Control aspects of a double-input buckboost power electronic converter

Systems in which two or more energy sources combine to supply power to a common load are called hybrid energy systems. Applications of these systems have grown due to their flexibility and reliability. Hybrid energy systems have been successfully implemented in hybrid electric vehicles and wind-solar systems where two or more energy sources share the same load. Double-input (DI) dc-dc power electronic converters (DIPECs) have been gaining popularity in hybrid energy systems due to their reduced component count and control simplicity. In addition, employing DIPECs increases the reliability, stability, and flexibility of the system. In this thesis, a small-signal model for one of the DIPEC topologies, the DI buckboost converter, is developed and compensator design is carried out based on the small-signal model. The compensators are designed to accommodate optimal power sharing between the sources. Theoretically, it is also proven in this thesis that the two inputs of the DI buckboost topology can be independently controlled which gives great flexibility in terms of the compensator design. Time domain analysis of the system is carried out with the compensators included and the results agree with the theoretical analysis. In addition to the small-signal modeling, a new control method called offset time control is also introduced and successfully applied to a DIPEC topology in this thesis. The control scheme is based on adjusting the offset time between the switching commands; which is proven to have a direct impact on the amount of current drawn from each input. Small-signal modeling of the offset time control scheme has been carried out to prove the improvement in the speed of response of the system when the offset time control scheme is applied --Abstract, page iii

Hardware integration of ultracapacitor based energy storage to provide grid support and to improve power quality of the distribution grid

Grid integration of distributed energy resources (DERs) is increasing rapidly. Integration of various types of energy storage technologies like batteries, ultracapacitors (UCAPs), superconducting magnets and flywheels to support intermittent DERs, such as solar and wind, in order to improve their reliability is becoming necessary. Of all the energy storage technologies UCAPs have low energy density, high power density and fast charge/discharge characteristics. They also have more charge/discharge cycles and higher terminal voltage per module when compared to batteries. All these characteristics make UCAPs ideal choice for providing support to events on the distribution grid which require high power for short spans of time. UCAPs have traditionally been limited to regenerative braking and wind power smoothing applications. The major contribution of this dissertation is in integrating UCAPs for a broader range of applications like active/reactive power support, renewable intermittency smoothing, voltage sag/swell compensation and power quality conditioning to the distribution grid. Renewable intermittency smoothing is an application which requires bi-directional transfer of power from the grid to the UCAPs and vice-versa by charging and discharging the UCAPs. This application requires high active power support in the 10s-3min time scale which can be achieved by integrating UCAPs through a shunt active power filter (APF) which can also be used to provide active/reactive power support. Temporary voltage sag/swell compensation is another application which requires high active power support in the 3s-1min time scale which can be provided integrating UCAPs into the grid through series dynamic voltage restorer (DVR). All the above functionalities can also be provided by integrating the UCAPs into a power conditioner topology. --Abstract, page iv

Power Sharing in a Double-Input Buckboost Converter using Offset Time Control

Multi-input power electronic converters have been gaining popularity in applications such as renewable energy sources and hybrid electric vehicles due to their reduced component count. In this paper, a new control method is introduced and successfully applied to a double-input buckboost converter to adjust the power supplied by each one of the sources. The control scheme is based on controlling the offset time between the switching commands while switching frequency is kept constant. Theoretically, it is proved that the offset time between the switch commands has a direct impact on the amount of current drawn from each source. The proposed control method has a very fast dynamic response and improves the stability of traditional controllers. Simulation results agree with the theoretical analysis

Study on the Effects of Battery Capacity on the Performance of Hybrid Electric Vehicles

Hybrid electric vehicles are gaining a significant presence in the auto market. However, the present day hybrid electric vehicles mostly use battery as a secondary source of power. If the battery were to be used as a primary source of power then the battery capacity is one of the important features in the design of a hybrid electric vehicle. Hybrid electric vehicles which are powered by more than one energy source have to follow a good energy management strategy to provide the best fuel economy in all situations. This paper presents a comprehensive study of the effect of variation of the energy storage system size on the fuel economy of a hybrid electric vehicle and the important design criteria involved in the design of the energy storage system. Simulations carried out using ADVISOR software show that increase in battery capacity alone cannot improve the fuel economy

Small-Signal Modeling and Analysis of the Double-Input Buckboost Converter

Multi-input dc-dc power electronic converters have been gaining popularity in applications such as renewable energy sources and electric-drive vehicles due to their reduced part count and flexibility in integration. In this paper, the small-signal model of the double-input buckboost converter is developed. The model is then used for a multiple loop feedback design. Considering a photovoltaic-battery hybrid power system, the control objectives are threefold. These include output voltage regulation, constant power demand from the PV panel, and load accommodation by the battery pack. Two feedback compensation networks are designed based on the developed small-signal model. It is also demonstrated that the two inputs of the double-input buckboost topology can be controlled independently. This offers greater flexibility for the compensator design. The results of the time domain analysis are consistent with those of the theoretical model

An Integrated Dynamic Voltage Restorer-Ultracapacitor Design for Improving Power Quality of the Distribution Grid

Cost of various energy storage technologies is decreasing rapidly and the integration of these technologies into the power grid is becoming a reality with the advent of smart grid. Dynamic voltage restorer (DVR) is one product that can provide improved voltage sag and swell compensation with energy storage integration. Ultracapacitors (UCAP) have low-energy density and high-power density ideal characteristics for compensation of voltage sags and voltage swells, which are both events that require high power for short spans of time. The novel contribution of this paper lies in the integration of rechargeable UCAP-based energy storage into the DVR topology. With this integration, the UCAP-DVR system will have active power capability and will be able to independently compensate temporary voltage sags and swells without relying on the grid to compensate for faults on the grid like in the past. UCAP is integrated into dc-link of the DVR through a bidirectional dc-dc converter, which helps in providing a stiff dc-link voltage, and the integrated UCAP-DVR system helps in compensating temporary voltage sags and voltage swells, which last from 3 s to 1 min. Complexities involved in the design and control of both the dc-ac inverter and the dc-dc converter are discussed. The simulation model of the overall system is developed and compared to the experimental hardware setup

Designing Efficient Hybrid Electric Vehicles

Hybrid electric vehicles (HEVs) are increasingly gaining popularity due to their lower fuel consumption. Current hybrid vehicles mostly use their battery or the energy storage system (ESS) as a secondary source of power. If the ESS were to be used as a primary source of power, then the ESS size would be one of the important features in the design of an HEV. In addition, HEVs have to employ an intelligent energy management strategy to provide the best fuel economy in all driving situations. This article presents an investigation on the effect of the variation of the ESS size on the fuel economy of an HEV and the important design criteria involved in the design of the ESS. Simulations carried out using advanced vehicle simulator (ADVISOR) software show that fuel economy is not linearly related to ESS size and therefore the ESS needs to be designed based on the average daily driving distance and the driver behavior

Power Sharing in a Double-Input Buck Converter Using Dead-Time Control

Multi-input converters are becoming important in various renewable energy applications like wind energy, fuel cell systems, photovoltaic systems, and hybrid electric vehicles. In this paper, it is shown that in a double-input buck converter the dead-time of the switch commands along with the two duty ratios will have a direct impact on the amount of current drawn from the sources/inputs. This dead-time is used as an additional control variable apart from the switch commands to meet the control objective of meeting a constant load demand when the source currents are varying which is very common in hybrid energy systems. Theoretical derivations agree well with the simulation results. The new control method has good dynamic response and improves the speed of the system