Making Solar Grid-Friendly

The world’s power generation systems continue to transition to cleaner, more renewable and sustainable sources. That effort will be greatly aided by integrated and comprehensive grid interconnection solutions. Utility-scale, grid-connected solar photovoltaic (PV), as well as wind, has become increasingly attractive as a generation resource, both in terms of economics and operational fl exibility. The technology needed to interconnect these renewable power sources is now well proven in the field.

 

Grid operators want solar to act like traditional power plants — to support stability and integrity during grid disturbances.

 

Following the rapid growth of the wind power industry in recent years, the solar power market is expanding on a global scale. A recent report by industry research firm IMS Research estimates that new PV installations grew last year by 130% — to 17.5 gigawatts (GW). The report predicts that total installed solar capacity will grow to as much as 58 GW by the end of 2011. IMS forecasts that the global PV inverter market will reach $8.5 billion by 2014, growing at a compound annual rate of nearly 25%. This rapid domestic growth is expected to propel the United States to overtake Germany as the world’s largest market for PV modules in 2014, according to the European Photovoltaic Industry.

But as an intermittent energy source, like wind power, utility-scale solar power plants today must meet stringent requirements for both reliability and grid-friendliness (grid management capability when interconnecting to the integrated, interstate transmission system. Such requirements have bee in place in Europe and Australia for quite some time, but have only recently become a reality for solar power plants in the U. S. and Canada. In light of its rapidly growing market, more stringent interconnection requirements are expected in North America for both wind and solar resources, as more megawatt-scale plants come online.

Realizing the full potential of solar power in the generation mix will require comprehensive wide-area solutions. One such solution would allow conversion systems for individual solar plants to stay on-line during grid disturbances to support and stabilize the grid system voltage by injecting or absorbing reactive power. Thus, true steady-state control of power factor and dynamic control should be seen as the hallmarks of special-purpose grid interconnection systems.

Interconnection Technology

Today, solar generating plants must meet power factor and low- and high-voltage ride-through requirements, as well as provide voltage control at the Point of Interconnection (POI). In addition, solar plants are required to act as a “good utility citizen.” That is, 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. Solar power plants should also 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.

As an intermittent resource, solar power has the same interconnection requirements as wind power. Utility-scale wind already amounts to approximately 40 GW of capacity in the U.S. alone. With the gaining of experience, the technical and operational issues related to grid interconnection have already been encountered and solved. Just as with wind farms, grid operators want solar plants to act like traditional power plants, and therefore maintain grid stability and support grid integrity during disturbances.

That means that solar plants must provide voltage support services, including defined quantities of reactive power, in order to maintain the voltage profile at the POI established by transmission operators. For example, based on our review of the most recent standards yet put forward, the Electric Reliability Council of Texas, Inc. (ERCOT), in Section 6.5.7, appears to be requiring a 0.95 leading to 0.95 lagging power factor, based on the nameplate rating of the generating unit (as opposed to the instantaneous output). In practice, this means a 40-MW solar plant would be expected to supply a little more than ±13 MVAR to the point of interconnection, even at very low power outputs. It appears that if the power output is below 10% of generating unit’s nameplate rating, then the requirement is relaxed, but the generator may be asked to disconnect. Note that this rule is specific to wind power and has not yet been applied to solar power.  However, a similar requirement for solar would be a next logical step. Due to the variable power levels stemming from both of these clean energy resources, it is clear that solar energy also will need to meet very specific interconnection requirements to ensure their transition to utility-scale operation.

The Basic Parameters

Connecting renewable power generation plants to the grid is a multi-disciplinary task. It involves a knowledge and correct interpretation of grid codes, experience in determining the correct equipment to be installed, and precise control of that equipment at the solar plant. For that reason, upfront planning for utility-scale solar power plants can help to establish grid stability.  Similar to the pattern that has occurred in the wind industry, electric utilities today are beginning to require greater grid-management support and reactive compensation from solar power plant developers. Developers, in turn, must meet these requirements while also keeping Balance of System costs as low as possible.

Solar plants should keep voltages smooth and stable, even if plant output varies due to passing clouds.

The optimal circumstance is for solar power plants to actively support the grid during power system disturbances. When disturbances do occur, interconnected solar plants can then assist with grid recovery and stay connected to the utility when they are most needed. This approach requires deploying dynamic reactive compensation, such as STATCOM devices.  Modern versions of STATCOM devices feature confi gurable (plant-specific) power factor levels and response times and have advanced support for unpredictable grid events such as sags or swells, frequency excursions, and transient high- or low-voltage conditions.

The most sophisticated solutions use solar inverters combined with reactive power elements and plant-wide controls, via a “smart grid” interface to provide very precise regulation at the POI. The control extends to dispatch of active/reactive power and management of ancillary switched capacitors/reactors, if needed, for the widest range of voltage regulation.

Special-Purpose Solutions

A key issue in today’s utility-scale solar space is the unfortunate reliance on interconnection equipment that needlessly limits the plant operator’s flexibility and also lowers potential plant availability.  For example, many solar inverters available on the market today are derived from small roof-top or commercial applications, which come with equipment that simply “disconnects” from the grid in the event of a relatively minor voltage disturbance. As a result, these inverters have no inherent capability for low-voltage ride-through (LVRT), 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 on account of voltage disturbances. A plant that trips off line will not be generating any revenue.What is preferable is a scalable solution for multi-MW solar plants that combines both real and reactive power building blocks along with effective controls technology.  A dynamic, multi-tiered approach provides the most economical pathway to meeting interconnection requirements. A grid-friendly plant is designed to stay on line during a grid disturbance, help the grid recover, and thus minimize disruption in revenue generation for the plant owner.

What is needed to meet stringent requirements for reliability and grid-friendliness is a comprehensive wide-area solution in which the conversion system stays on-line and further acts to support and stabilize the system voltage by injecting or absorbing reactive power.  True dynamic voltage control and steady state control of power factor are the hallmarks of such special-purpose grid interconnection systems.

For example, a “smart grid interface” could measure the voltage and current at the Point of Interconnection to determine and manage the power quality of the system. When a sag or swell event occurs, this interface would command inverters in the plant to provide the needed reactive support to control the voltage at the POI. If necessary, the smart grid interface would command additional reactive support from STATCOM and/or static shunt devices, if installed as part of the system.  The best solution is for the smart grid interface to actively know the reactive power capability of the system for both fast and accurate responses. With such a solution, variances imposed at the POI — due even to passing clouds — can be compensated for via reactive support to control the POI voltage within the range delimited by the interconnection agreement.

ABOUT THE AUTHOR: Perry Schugart serves as Director, Power Converter Business, Power Systems Business Unit, at American Superconductor (www.amsc.com), having joined the company is December 2001 as Director of Sales and Marketing of the company’s Power Electronics Business Unit. Prior to joining American Superconductor, Mr. Schugart held increasingly senior positions with International Rectifi er, most recently as Director of Sales Development. During his cumulative 10 years at International Rectifier, Mr. Schugart oversaw tremendous growth in the company’s sales in both the proprietary and advanced technology products, and was an integral in the integration of multiple acquisitions. An Illinois native, Mr. Schugart holds a Bachelor’s of Science Degree in Physics from the University of California, Santa Barbara. Contact Mr. Schugart at pschugart@amsc.com.