By Perry Schugart

The composition of the world’s electric generation portfolio is changing. Non-fossil-fuel-based energy supplies, such as wind, solar, and biomass, are increasingly sought after as a means to reduce the environmental impact of energy production.

However, grid interconnection has been identified as one of the most--if not the most--significant barriers to the installation of distributed generation technologies. To overcome this barrier, various requirements and standards have been implemented for interconnection. This is particularly true in the wind industry, which has 197 Gigawatts (GW) of global capacity at present. Unfortunately, these requirements vary by states, regions, countries and utilities, creating a complex patchwork of interconnection requirements.

Therefore, the most effective strategy for developers has been a two-pronged approach: 1) design to the most demanding interconnection standard--regardless of the source of energy and 2) employ a grid interconnection expert to ensure the lowest cost of connection.

Utility-Scale Solar Demand Is Growing Rapidly

Worldwide, the solar power industry is booming and the market for utility-scale solutions is forecasted for particularly impressive growth. In fact, industry research firm IMS Research expects annual shipments of solar Photovoltaic (PV) inverters grow from approximately 828 MW at the end of 2009 to nearly 11 GW in 2015. The utility-scale market is expected to grow at a faster pace than the commercial and residential markets.

While this is certainly a positive for all involved in this industry, the rapid growth of utility-scale solar also signifies that grid interconnection requirements--like those for the wind industry--are most likely pending and therefore, developers need to plan accordingly.

What’s more, utility-scale installations are more cost effective than residential and commercial projects and, with the correct interconnection planning and equipment, can provide huge economic and environmental benefits.

Like Wind, Solar Plants Should Act Like Traditional Power Plants

Utility-scale solar power is experiencing challenges similar to wind power when connecting to the grid. Grid operators want solar plants to act like traditional power plants, and, therefore, maintain grid stability and avoid blackouts. Like wind power plants, solar power plants require reactive compensation, and rigorous reactive compensation standards are likely to become a reality for solar power plants in North America, Europe, and Asia.

Upfront planning for renewable energy power plants can help to not only establish/maintain grid stability, but also to lower the cost of energy. Connecting renewable power to the grid is a task that involves the knowledge and correct interpretation of grid codes, expertise in connecting renewables to the grid, experience in determining the correct equipment to be installed, and the precise control of that equipment at the solar plant.

The bottom line is, due to the successful field experience gained through the interconnection of numerous wind plants, the solar industry will have a leg up on the wind industry and will be able to quickly and efficiently deploy interconnection technology sophisticated enough to meet any conceivable set of standards. This is exemplified in the case of American Superconductor (AMSC). AMSC is well-versed in grid interconnection technologies because it is today connecting over 15 GW of renewable energy (the majority of that wind) to the grid. AMSC has transferred this knowledge to the utility-scale solar industry, and built its SolarTie™ Grid Interconnection Solution by combining two of its proven and proprietary technologies: D-VAR® STATCOM solutions and PowerModule™ power converter systems. Indeed, on this basis, we can say that the ‘future’ of solar grid interconnection technology has already arrived.

Grid Interconnection Technical Requirements

Despite the technical complexity of evolving interconnection requirements, there are commonalities in the major jurisdictions. In general, global interconnection standards consist of the following subsets:

1) Reactive power requirement (power factor), both continuous and dynamic. The definition of ‘dynamic’ changes grid code to grid code, but generally means fast acting, continuously variable (linear output), short term VAR support;

2) Voltage Ride Through (LVRT/HVRT/Stability), where the renewable plant (wind or solar) must both survive credible contingency events on the grid (voltage sags or swells) as well as contribute to post-fault recovery, and may also be required to actually support the grid during contingency events; and

3) Requirements specific to the local utility, including control mode, point of regulation, etc. Typically, these are defined in a System Impact Study (SIS) conducted by the utility/system operator.

In practice, renewable resources have come to be built much faster than the required transmission upgrades can be designed, approved and built. New transmission and transmission upgrades are essential to link these location-constrained renewable facilities to the backbone power grid. New strategies, including much more sophisticated and comprehensive interconnection solutions, are needed to manage the transmission network to facilitate the maximum delivery of renewable energy to customer loads.

The output from wind and solar generation can change dramatically, both up or down, in a very short period of time. These rapid changes pose a significant challenge to system stability and to the grid operator who must meet reliability performance standards. In the case of wind, this has meant that wind generators must typically meet low voltage ride through standards, voltage control, and other large generator interconnection standards. Regardless of regulatory requirements, the trend in the industry is to move toward adoption of the best available technology, while minimizing lifecycle costs.

In the absence of clearly defined interconnection standards, performance guidelines will be defined during the system impact assessment conducted by the local utility/system operator. Herein lies perhaps the greatest contribution of the SolarTie interconnection solution because not only does it accommodate the major aspects of worldwide grid codes--including LVRT, reactive power support, and various control modes (such as voltage control, power factor control, and constant susceptance)--technically it can meet any conceivable set of operating parameters enforced by local system operators. Additionally, the SolarTie system will be able to control multiple VAR assets (such as D-VAR STATCOM, shunt caps/reactors, etc.) in addition to the solar inverters themselves with a single centralized control system.

A Megawatt-Scale Solution For a Multi-Megawatt-Scale Plants

Because the commercial and residential solar markets are more mature than the utility-scale market, utility-scale plants in today’s solar market have unfortunately been reliant on equipment that needlessly limits the plant operator’s flexibility and also lowers potential plant availability. What is preferable is a scalable solution for multi-MW solar plants that combines real and reactive power building blocks. A dynamic, multi-tiered approach, like the SolarTie, which is specifically designed for megawatt-scale solar PV power plants, provides the least cost path to economically meet interconnection agreements.

Many solar inverters available on the market today are derived from small rooftop or commercial applications for which the equipment is simply required to ‘disconnect’ from the grid in the event of a relatively minor voltage disturbance. As a result, these inverters have no inherent Low Voltage Ride Through (LVRT) capability, a key and basic interconnection parameter, and one that will undoubtedly be made stricter in the years to come. While some of these inverters come with a ‘software switch’ that disables the automatic tripping of the inverter during low/high voltage events (as long as the voltage stays within some safe operating limits of the unit), a grid-friendly low/high voltage ride through solution has to do more than just simply disable the inverter’s tripping upon voltage disturbances. Furthermore, the solution applied must be flexible enough to allow further changes in the grid requirements.

Solar power generating plants today must meet power factor and low and high voltage ride-through requirements and provide voltage control. They additionally are being required to be a ‘good utility citizen’ as part of a growing renewable portfolio, meaning they must first be grid-friendly and cannot disconnect from the utility when they are most needed such as during power system disturbances. Secondly, they must actively support the grid so that when disturbances do occur, the plant is prepared to help the grid recover. Finally, the solar power plant should provide day-to-day voltage support to help keep the system voltages smooth and stable, even if the power output of the plant varies due to clouds or other factors during the course of the day.

Residential or commercial sized inverters are simply not robust enough to meet these requirements. Realizing the full potential of solar power in the generation mix will require comprehensive wide-area solutions. These solutions will enable individual solar plants’ conversion systems to stay on-line to support and stabilize the grid system voltage by injecting or absorbing reactive power. Thus, dynamic control of voltage and frequency are the hallmarks of megawatt-scale grid interconnection systems. In the most technically desirable solution, a ‘smart grid’ interface, like that in AMSC’s SolarTie, can respond to external commands to set the power factor at the Point Of Interconnection (POI). Such an interface could control the reactive support level of individual plant inverters and command additional reactive power devices, if installed as part of the system, to achieve the desired steady-state control objective at the POI. Transient voltage control, with sub-cycle detection and response to grid disturbances, is enabled at the POI. With such a solution, variances imposed at the POI due even to passing clouds can be compensated for via reactive support to precisely control the POI voltage within the range delimited by the interconnection agreement.

While wind and solar installations are intermittent in nature, the goal of system operators is usually to have similar reactive power and/or voltage support capability as a conventional generator such that the existing power system is not adversely affected by the varying power generation. Dynamic reactive capability for utility-scale renewable generation facilities is considered essential. In general, it is expected that the solar industry will follow a similar path as wind, adopting similar technical and performance standards. Additionally, much like wind standards, solar installations should expect technical standards to be somewhat fluid early on, as the utility market adapts to elevated levels of solar penetration. This being the case, it should be emphasized that stringent performance standards such as LVRT, stability, voltage support, varying control requirements, etc. seen in wind applications will be applied to solar applications, and it, therefore, may be prudent for solar developers to consider these requirements during the design/planning stages and equipment selection.

Perry Schugart is Director of Power Converter Business, American Superconductor (AMSC) Power Systems (www.amsc.com). Schugart holds a Bachelor’s of Science Degree in Physics from the University of California, Santa Barbara. He joined American Superconductor in December 2001 as Director of Sales and Marketing for Power Electronic Systems. In 2002, he was appointed to the position of Director of Power Converter Business, AMSC Power Systems.