Development of interface for grid integration of energy storage systems in the built environment

• Main Author: Hu, Xiaolei
• Other Authors: Madhavi Srinivasan
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Electric power::Auxiliaries, applications and electric industries
• Online Access: https://hdl.handle.net/10356/62244
• Institution: Nanyang Technological University

Ac power grid in any building in future will increasingly have to accommodate a growing number of energy storage systems (ESSs) incorporating with renewable power generation. Integrating ESSs into buildings can improve energy efficiency and sustainability; and also provide a usable power source during utility ac grid fault. Batteries and super-capacitors are considered as excellent candidates for energy storage devices (ESDs) used for ESS. Batteries have good energy density and super-capacitors have good power density. However development of control strategy and electrical interface for integrating ESS into utility grid of green buildings is still a challenge. Using only either batteries or super-capacitors as ESDs cannot guarantee an optimal system performance since batteries have limited power rating while super-capacitors have poor energy density. Hybrid system of batteries and super-capacitors has a compromising system performance. However, conventional control method using first-order low pass filter (LPF) or PI controller doesn’t optimally regulate current stress and charging/discharging cycles of batteries. Model predictive control (MPC) is introduced and designed in this thesis to minimize current stress and charging/discharging cycles of batteries to extend battery life. Another challenge of developing ESSs for green buildings is how to build proper power converters to interface battery or super-capacitor cells that only have low terminal voltage to utility grid that has much higher voltage. Cells of ESDs can be connected in series to obtain sufficient terminal voltage and a bidirectional dc/dc converter links the series-connected cells to a high voltage dc bus. Cascaded voltage source inverter (VSI) can interface the dc bus to utility ac gird of green buildings. However, due to varying chemical and electrical characteristics, series-connected cells have unbalancing issue after several charging/discharging cycles. Battery cell equalizer (BCE) is used for balancing series-connected battery or super-capacitor cells as an auxiliary circuit. In application of green buildings or electrical vehicles, fast cell equalizing is desired. A current-fed BCE is proposed in this thesis for applications that require fast cell equalizing. Modularized design of ESS has high flexibility and reliability. One common application of ESS used in real-world is uninterruptible power supply (UPS). Dc bus overvoltage is observed in the investigation of paralleled modularized UPS. This instability issue is also shared by application of renewable power generation. This thesis studies the cause of this dc bus overvoltage. It was found that circulating current at switching frequency caused active power exchange and thus caused the dc bus overvoltage issue. A novel switching frequency shift method is proposed in to suppress the dc bus overvoltage. Another way of interfacing ESDs to utility grid is to use power electronics converters with high voltage transfer ratio. Dual active bridge (DAB) converter plus VSI has been proposed by researchers recently. DAB has bidirectional power flow capability and the feature of zero voltage switching (ZVS). The system can be optimized when the voltage transfer ratio of the input ESDs dc voltage over dc-link voltage is a constant. However DAB has high circulating current when conventional control method is adopted, e.g., traditional phase shift (TPS) control. A hybrid control method of phase shift and PWM is proposed and introduced in this thesis to eliminate the circulating current of DAB converter. Comparisons of the proposed hybrid control method to TPS and dual phase shift (DPS) are present in this thesis. Some ESDs have wide varying terminal voltage and bring challenges to the design of using DAB+VSI since dc-link voltage may be quite high when the voltage transfer ratio of the input ESDs voltage over dc-link is constant. A novel converter based on current-fed topology and current source inverter (CSI) is proposed in this thesis. The proposed circuit has a boost function from the ESDs with low voltage to ac grid with high voltage. Therefore, the proposed converter is compatible with ESDs having wide terminal voltage. Furthermore, dc-link capacitors used in DAB+VSI and grid side current sensors can be eliminated. Circuit operation and modulation of the proposed converter is explained in this thesis.