Project: Improving Power Quality and Safety Operation of Multiple Grid-Connected Residential Photovoltaic (PV) Systems with Distributed Storage, Control and Power Conditioning Systems

 

Project Objective: Promoting the higher penetration of photovoltaic (PV) generation will require further installation in the residential sector. The power quality problems such as voltage rise caused by reverse power flow, current harmonics, and lack of local reactive power compensation are mainly found in projects with large penetration of PV on rooftops of houses and commercial buildings. Another problem is the unintentional islanding prevention for multiple PV systems.  Because these problems can adversely affect the electric power quality of the distribution network and safety operation, these issues are significant causes to restrict the expansion of the multiple PV systems. Therefore, the project objective is to develop effective methods and technology to resolve the above problems so that a power system composed of hundreds of residences, each installs a PV system, can benefit from this research.

In this project, a single phase cascaded grid-connected PV system integrating segmented batteries is proposed as shown in Fig.1. Hybrid phase-PWM technology is conducted to reduce the switching loss and improve the dc boost ratio. The proposed system can achieve: (1) reduced energy conversion stages; (2)effective PCC overvoltage mitigation due to appropriate power management between PV and batteries; (3) smooth PV power to enhance system stability and reliability; (4) enhanced real power allocation and wide range reactive power compensation to reduce the system burden and improve power quality. As shown in Fig.1, the “main” inverter and “auxiliary” inverter cells are connected to PV and batteries, respectively. The voltage ratio between Vdc, VES1 and VES2 is 2:1:1. The research has revealed that the cascaded auxiliary cell number of 2 and selected voltage ratio are optimized considering the trade off among the cost, power quality and reactive power compensation capability. A hybrid modulation strategy combining SPWM with phase-shift control is adopted for the system, the “main” and “auxiliary” inverters switch at fundamental and PWM frequency, respectively. The cascaded multilevel inverter is connected to grid through a LCL filter (L1, Cf, L2), which is used to reduce the lower switching frequency current ripple and maintain good system dynamic response. Lg and Rg are the transmission line impedance between PCC and grid, which are used to investigate the power flow effect on PCC voltage. P_grid and Q_grid are P and Q from/to the grid. P_load and Q_load are P and Q to the load. P_main is unidirectional P and Q_main is bidirectional Q from/to the main inverter cell. Q_auxi1 and Q_auxi2 are bidirectional Q from/to two auxiliary inverter cells. P_auxi1 and P_auxi2 are bidirectional P provided or absorbed by two auxiliary inverter cells. The PV system is able to operate in stand-alone mode and grid-connected mode through a static transfer switch (STS).

　

References:

Yan Zhou, Liming Liu, Hui Li, “Autonomous Control Integrating Fast Voltage Regulation and Islanding Detection for High Penetration PV Application,” Proc. 26th IEEE Applied Power Electronics Conference and Exposition, (APEC’11), Fort Worth, TX, USA, March. 2011

　

CONTACT:                                      SPONSOR:

Dr .Hui Li, Associate Professor          NSF Award number: 1001415

hli@caps.fsu.edu  (850) 644-8573