Bringing Utility-Scale Solar Power to the Grid

Utilities are being challenged to embrace this rapidly evolving wave of change. Some have concerns that integrating a high volume of inverter-based photovoltaic systems and other distributed generation sources will lead to instabilities and the possibility of unsafe grid operations. Variable energy production and the fact that peak production from these sources does not always coincide with peak demand can reduce the value of PV’s impact on utility operating economics.

The impact of distributed production on fault-protection and system repair safeguards can also be significant. These are valid concerns. It is clear that the old system of “one-way” power flow will not be sufficient in the future. A new paradigm of integrated systems offering two-way power, control and information sharing is required. Not only will the technical issues have to be solved, but utilities will have to adjust their historic view of the grid architecture to embrace distributed generation and work with the other parties involved to create an optimized solution.  (Left: This 100 kW installation by the Oregon DOT is one of the country's first solar highway projects.  Courtesy PV Powered.)

DOE Involvement

Realizing the magnitude of the problems to be solved, the U.S. Department of Energy (DOE) has initiated the Solar Energy Grid Integration System (SEGIS) project. SEGIS brings together utilities with leaders in the field of photovoltaics, energy management and communications to develop the new products and technologies necessary to achieve high penetration of PV systems into the utility grid. Outcomes from this project will include advanced, highly-integrated inverters; new systems of communication; and less costly, more reliable system components, which will easily accommodate the two-way power and information flows required for seamless integration.

In the current Phase 2 or “Design Phase” of the SEGIS project, five teams composed of industry-leading companies from across the country are working on various aspects of the project. PV Powered Inc., a Bend, Ore.-based maker of solar power inverters, leads a team charged with addressing the project’s core concerns: utility integration and control; system cost, reliability and efficiency; and integration with building monitoring and control systems.

As an increasing number of utility-scale PV power plants are being connected to the grid, problems are being identified. These relate to the distributed PV resource’s intermittent nature and the inherent conflict between a power generation source and existing grid interconnect standards governing distributed PV system connection to the grid.

Both problems are complex and their final solution may be realized only when interconnection standards have been changed to embrace PV as a key energy generation asset for utilities. Additionally, smart grid communications infrastructure will likely be required to fully solve these problems at a level that addresses high PV penetration in the case of highly distributed solar generation.

As first steps toward this goal, SEGIS team members PV Powered and Portland General Electric (PGE) have integrated two-way communications between the solar power plant and PGE’s GenOnSys distributed supervisory control and data acquisition (SCADA) system. This enables the utility to receive status information and assert control commands as necessary, including disconnecting its fleet of distributed resources remotely if needed.

As PV penetration increases, the problem of how PV systems detect and react to grid variations becomes increasingly critical to overall grid stability. More interactive controls are required to ensure that inverters will disconnect when necessary, but will be able to stay on-line when drops in utility voltage and frequency levels occur. PV can assist in riding through these temporary fluctuations. This function is typically implemented by a sophisticated set of algorithms that perform passive monitoring and active control within an inverter to determine if an unintentional island has been created (where the PV system sends power into a section of the utility grid experiencing an outage). Present-day inverters cannot differentiate between a true utility outage (where anti-islanding is necessary) and a grid disturbance or brownout in which the PV system could actually help support grid stability. Even if these inverters could differentiate between these conditions, current regulations sometimes require the inverter to disconnect from the grid when additional power is most needed.

A better method for island detection is being developed by the SEGIS team that PV Powered is involved with. This team is using a pioneering application of synchrophasor measurements to enable the inverter to differentiate between a true unintentional island and a situation where grid support from the PV plant is required.

Synchrophasor measurements are taken at different locations in a power system using the same absolute time base. This provides an accurate and reliable method of correlating values from various locations that take different amounts of time to arrive at a common collection point.

To accelerate PV penetration, it is essential that the cost of energy from PV systems continues to decline when compared to conventional sources. Here, cost is broadly defined to encompass not only initial cost but reliability, energy harvest and overall lifecycle costs as well embracing the goal of achieving the lowest total cost per kilowatt-hour over a system’s lifetime.

The inverter/controller is the heart of a PV system. As PV penetration expands and production volumes rise, the cost of inverters is coming down. However, cost is only one part of the equation — inverters can also decrease the lifetime cost per kilowatt-hour (kWh) by offering better performance, higher reliability and more integrated features that improve energy harvest.

System Reliability

High reliability is a key part of managing overall lifecycle costs. Frequent service calls and repairs or system component replacements can significantly reduce a system’s value. Some proposals being explored to improve reliability include expanded use of integrated circuits, thermal management, surge protection, self-diagnostics, reduced overall parts count and eliminating the least reliable components or using selective redundancy to ensure inverter uptime. Additionally, data aggregation and analysis protocols are being developed to improve reliability predictions for individual components and each system as a whole. Finally, new design features are being implemented to reduce the cost and complexity of installation and servicing.

Harsh environmental conditions further tax PV system reliability. Hydroelectric, nuclear and coal- or gas-fired power plants typically reside in a controlled environment such as a building. By contrast, most solar PV power plant components are directly exposed to the outside environment, subjecting them to temperature fluctuations and extremes, humidity, corrosives, dust and other location-influenced stresses. All this must be factored into any reliability analysis. To accurately predict solar inverter component stresses and associated wear-out mechanisms due to natural cycles, a complex time-dependent modeling approach is required. Because temperature cycling contributes to device wear-out, simpler constant hazard rate calculations that might apply in other situations often are not accurate in this case. PV Powered has created a set of time-dependent prediction tools and analytical methods to predict real-world inverter reliability with greater accuracy and granularity than methods commonly used today. (Above: A mobile solar cart used to test different array technologies as part of the SEGIS development project. Courtesy PV Powered)

Ensuring maximum energy harvest is a function of a number of factors, including a given system’s efficiency, reliability and uptime and the system’s ability to adapt to dynamically changing irradiance conditions. Within an inverter, the maximum power point tracking (MPPT) algorithm (which varies the ratio between the voltage and current delivered by a solar array to deliver maximum power as the array output changes) is a key factor in maximizing overall solar power plant efficiency. As inverter power conversion efficiency from the arrays nears theoretical maximum, the accuracy and efficacy of the MPPT algorithm emerges as one of the few remaining high-value opportunities to increase total energy harvest. Quantifying MPPT efficiency and developing a new MPPT algorithm that provides highly accurate tracking efficiency over static and dynamic irradiance conditions is a challenge in terms being able to adapt MPPT behavior to various PV materials and fast changing environmental conditions. PV Powered is testing an MPPT algorithm that may deliver superior performance under a variety of conditions and PV materials.

Another key factor in improving energy harvest is managing weather-related irradiance transients. Unlike other forms of power generation, a solar power system’s inputs are inherently variable due to weather and fluctuations in cloud cover. Without active management power output to the grid can be highly variable and disruptive. This is one of the main barriers to high-penetration PV. Use of irradiance forecasting can mitigate the effects of irradiance transients. SEGIS research is working to develop forecasting methodologies at both the utility level (where forecasting can allow more optimal integration into utility real-time dispatch processes) and the inverter level, where timely insights into cloud position, movement and transparency may be used to “soften” any transients the utility sees.

From small residential systems to solar farms and utility-scale installations, PV power systems have continued to mature and expand to the point where they are a viable part of the distributed-generation energy future. While significant challenges to successfully implementing high PV power penetration onto the U.S. grid remain, through collaborative efforts such as the SEGIS program, teams of industry and utility experts are working through the issues and developing the technologies that will enable a bright future for distributed utility-scale solar power.

Tucker Ruberti joined PV Powered in 2007 and is director of product management. Mr. Ruberti earned a BS in Industrial Engineering from Cornell University and an MS in Environmental Management & Policy from the Rensselaer Polytechnic Institute. He has worked for a range of companies including Westinghouse, General Electric, the New York State Energy Research and Development Authority, Sunlight Solar and IdaTech.

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Sidebar: Solar Energy Grid Integration Systems Projects

Through industry partnerships, the Solar Energy Grid Integration Systems (SEGIS) initiative is developing advanced systems and technologies to enable the electric power grid to handle large amounts of photovoltaics.

In 2008, the U.S. Department of Energy Solar Energy Technologies Program (SETP) awarded 12 companies financial assistance to develop a smarter grid. The projects receiving funding include more interactive components and systems that will help solar energy achieve grid parity by 2015. Awardees included Apollo Solar, Edison Materials Technology Center, Enphase, General Electric, NexTek Power Systems, Petra Solar, Premium Power, Princeton Power, PVPowered, SmartSpark Energy Systems Inc., Grid-Smart Inverters (University of Central Florida) and VPT Energy Systems.

As recipients of Solar Program funding, each company completed Stage 1 conceptual designs and market analysis. Each project then was evaluated prior to receiving a second round of funding on the likelihood of its success and the potential for commercialization. In 2009, five companies were awarded additional funding. These were:

• Apollo Solar: Advanced Grid-Tied Inverter, Charge Controller, Energy Monitor and Internet Gateway
  Developing advanced modular components for power conversion, energy storage, energy management and a portal for communications for residential-size solar electric systems. Pursuing inverters, charge controllers and energy management systems that can communicate with utility energy portals for implementing two-way power flows of the future.
• Petra Solar: Economically Viable, Highly Integrated, Highly Modular SEGIS Architecture
  Advancing grid interconnection coupled with lower costs, higher system reliability and safety through low-cost, easy-to-install modular and scalable power architectures. Developing multi-layer control and communication with PV systems to achieve monitoring and control for a cluster of alternating-current module inverters integrated with a strategic energy management system box.
• Princeton Power: Demand Response Inverter
  Designing innovative commercial-scale demand-response inverter, based on a new, unique circuit and new material, component, and packaging technologies. Developing optimized design for low-cost, high-quality manufacture that will integrate control capabilities (that is, dynamic energy storage and demand-side load response).
• PV Powered: MPPT Algorithms, EMS Integration and Utility Communications Advancements
  Creating a suite of maximum power point tracking algorithms to optimize energy production from the full range of PV module technologies. Integrating communications with facility energy management systems and utility management networks.
• University of Central Florida, Grid-Smart Inverters:
  Researching concepts to enhance intelligent grid development with PV that incorporate optional battery storage, utility control, communication, monitoring functions, and building energy management systems. Developing anti-islanding strategy for PV inverters to allow PV generation to remain connected during some grid disturbances, while meeting safety operation requirements. Designing new inverter architectures that bring more stability to the grid.