Grid Interface

The grid interface area addresses the major challenges associated with conversion, conditioning and control of electric energy from renewable resources such that these resources can be reliably and efficiently integrated to the electric power grid or to support other stand-alone loads. Solar photovoltaics and wind, two of the most important renewable resources, both require power electronics based interface to interconnect to the grid – dc-ac conversion in the case of PV, and variable speed generator control in the case of wind.

Some of the focus areas include: Research on high performance power electronic converters and control methods, design of power distribution systems that enable very large scale integration of distributed, renewable resources, interconnect techniques that enhance the reliability, availability and power quality of electric grid while simultaneously enhancing the value of renewable resources, optimal storage and storage management.

The Electric Power and Energy Systems group involved in grid interface research is one of the largest and most reputed power engineering programs in the country with more than 60 active researchers including faculty members, post doctoral scholars and graduate students. ASU is the lead school of a large NSF IUCRC - Power Systems Engineering Research Center (PSERC), which is a consortium of13 universities and about 40 industry partners. ASU is also part of a recently created NSF Engineering Research Center – Future Renewable Electric Energy Distribution and Management (FREEDM), which aims to develop technologies to revolutionize the nation's power grid and speed renewable electric-energy technologies into every home and business. ASU has well equipped and modern research laboratories for power electronics, high voltage and instrumentation, as well as a complete portfolio of power system modeling, analysis and simulation tools.

Power conversion

Power electronic converters represent an essential subsystem in the utilization of several renewable resources including solar PV and wind. They are also essential for integrating energy storage with renewable resources and grid. The power converters have a significant impact on the reliability and performance of the overall renewable energy system.

The main research themes at ASU in this area power conversion are:

Novel topologies: We are developing new circuit topologies, particularly well suited for PV power conversion, with the main goals of enhancing reliability and efficiency. For example, we are investigating multi-port, active-bridge based bi-directional configurations that can optimally interface several dc sources, batteries and the power grid/loads, and current source converter topologies that have higher conversion efficiency. One of the weak links that degrade the reliability and life time of present PV inverters is the large electrolytic capacitor. We are investigating new topologies that eliminate the need for large electrolytic capacitors. We are also developing circuit features that enable several value added functionalities that the future power converter will be required perform.

High performance PWM methods: The switch mode power electronic converters used to interface renewable resources inject high frequency currents into the power system. There are several standards that specify stringent restrictions on the amount of high frequency currents injected, in order to ensure the power quality of the grid. Large filters are used to meet these requirements, which increase the cost and size and also compromise system dynamic performance. As the penetration of distributed renewable resources increases, this problem will be highly exacerbated. We are investigating new pulse width modulation techniques that can significantly reduce the high frequency ripple without the need for large filters or very high switching frequencies. For single phase converter, we are analyzing variable frequency PWM methods, and for three phase converters we have developed advanced, space vector based digital PWM methods employing novel switching sequences.

Converter control techniques: Design of stable and high performance controllers for PV integration becomes a major challenge when distributed PV becomes ubiquitous. We are developing accurate models for the distribution system under different operating and fault conditions, and high performance, digital control methods based on these models. We are also interested in control algorithms for optimal storage management and prognostic methods that continuously monitor the health of critical components to improve system reliability.

A major research theme for the power electronics group recently has been the fully modular power conversion architecture. In this architecture, low voltage and low power building block modules can be configured in any combination of series and parallel connections at the input and/or output sides, at the dc and/or ac terminals with active sharing of voltage, current and power among the modules.

Control

Widespread use of distributed PV generation poses several challenges to the design of controls in terms ensuring that the thousands of PV power converters in close proximity interact stably with each other and with the electric grid. Also, the future power electronic based interface systems will be required to perform several value added functionalities in addition to basic dc to ac power conversion. Some of these functionalities include providing on-demand reactive and active power support to the grid, demand management, peak shaving, storage management and power quality. In order to meet these requirements and to enhance the value of PV, the concept of universal power management system (UPMS) is proposed and is being actively investigated.

ASU power electronics lab has significant expertise and experience in digital control of power converters using several digital signal processor (DSP) and FPGA development systems. A majority of the hardware prototypes built here for applications ranging from low voltage dc-dc converters in voltage regulators for microprocessors to unity power factor rectifiers and dc-ac converters for PV interface employ digital control and digital power management. The use of digital control in these applications enable implementation of complex linear, non-linear and hybrid control algorithms as well as advanced PWM implementations such as optimal, variable switching frequency schemes.

A main requirement for developing high performance control methods is the availability of accurate and validated models for the power distribution systems in the context of large scale distributed generation. Hence, a main research effort at ASU focuses on developing suitable models of distribution systems under different operating conditions and different fault scenarios. These models are designed to incorporate varying degrees and different types of renewable generation. In addition, a wide range of simulation tools are under development ranging from full-fledged switching models to simplified average models to phasor models that capture just the critical, low frequency information but capable of achieving several orders of magnitude reduction in simulation time for very complex systems.

Interconnection

Interconnection of renewable resources to the grid introduces significant issues related to reliability, security, safety, metering and power quality of the grid. We study the integration of large scale renewable resources especially wind, concentrated solar and PV, at the high voltage transmission and sub-transmission systems, as well as widely distributed integration at the lower voltage distribution and residential power systems.

Some of the specific research themes that we are currently pursuing include:

High voltage transmission systems to support bulk renewable energy integration and transport: The bulk transmission network currently is operating close to its limit in several portions of the country. With the addition of new renewable energy sources and with the added need to provide geographical diversity for intermittent sources like wind, it has become imperative to site and design bulk transmission networks to provide reliability and efficiency. Large scale storage in conjunction with bulk transmission also plays a critical role in enhancing reliability and efficiency. These are two topics currently being investigated by the group.

Networked distribution systems: Present distribution systems are predominantly radial, where the power flows from a fixed source end to downstream loads. As the percentage of distributed generation using renewable resources becomes significant, the unique requirements of distributed generation such as bi-directional power flow and intermittent generation begin to pose serious problems for the radial distribution system. Hence, we are investigating optimal, networked design of distribution systems with emphasis on proper protection coordination, safety and reliability of power. We will rely increasingly on extensive communication infrastructure for grid operation as well as other emerging smart grid technologies.

Solar residential microgrid: We are investigating the novel approach of residential microgrid as an attractive solution to simultaneously enhance the value of PV and ensure the reliability of the power grid. The concept is that a large, master planned community of several thousand homes is fitted with PV system and battery storage in a majority of the houses, forming a microgrid. The microgrid is connected to the grid at a single point through a disconnect switch; under normal operation it is connected to the grid and energy transactions between the grid and the microgrid is controlled based on real time pricing and need for real and reactive power support for the grid. During faults or other problems in the main grid, the microgrid disconnects itself, and the residential community is supported totally by its local generation and storage. Our research focuses on control methods for the microgrid controller and the individual power management systems in each house, the extensive communication requirements, and intelligent storage management. Another main objective of the research is to investigate the impact of widespread adoption of plug-in hybrid electric vehicles (PHEV) on the proposed solar residential microgrid.