Renewables challenge transmission and distribution paradigms
By Nicholas Abi-Samra, IEEE smart grid and asset management expert
Now that a renewable energy future is on the horizon, how are we going to handle it?
As renewable energy becomes a large percentage of total energy produced globally, its impact on the quality and reliability of the electric power grid have become more pressing -- especially for the utilities that must deal sooner rather than later with the power quality problems renewables can create.
The once theoretical question, "What happens when one-half of a utility's generating capacity depends on whether the sun is shining?" has already become a very practical one because electric utilities operating in a regulatory environment cannot deploy solutions in short timeframes.
There is no lack of technology solutions but few utilities know what mix of solutions is best for them because they do not have a detailed understanding of the specific challenges renewables will create in a particular service area.
When a specific renewable energy source is both large and well-defined -- such as a large wind farm -- power can be conditioned at or near the source. The situation is very different when a very large number of small sources -- such as rooftop solar panels -- are pumping varying amounts electricity into or out of the grid from sites that are, as a practical matter, invisible to the utility.
The time for definitive plans is fast approaching. In 2011, about one-fifth of power generation capacity worldwide was supplied by renewables. Investment in renewable energy is growing. More than 200 gigawatts of renewable electric capacity was added globally during 2011 alone, bringing the cumulative amount of capacity from renewables to 1360 GW.
Distribution Systems Challenges
Renewables can introduce variability, intermittency and fluctuations in the utility's distribution system. Large installations such as wind or solar farms can have these negative effects when they constitute a sizable percentage of total power produced. But even when some of the penetration levels are around 10 percent, some concerns have been reported. Typically, solar or photovoltaic generation is the most problematic.
The most commonly reported problem associated with high PV penetration on distribution feeders is high steady-state voltage. Severity depends on the feeder characteristics and location of the PV generation along the feeder. The impact is typically reduced as the distance of the renewable power source to the substation decreases.
Now that a renewable energy future is on the horizon, how are we going to handle it? ___________________________
Several solutions are available, including more accurate and timely forecasting, which can make hardware-based systems more effective. A number of hardware solutions are available. They can broadly be divided into two groups: reactive power control and energy storage.
Today, utilities use reactive power control primarily for power factor correction. But this technology can also play an important role in controlling voltage levels. VAR (Volt Ampere Reactive) devices inject energy to smooth the swings in supply and keep the voltage at acceptable levels. The DC/AC inverters that connect PV to the grid, on the other hand, can serve as controllers because they can technically consume or provide reactive power quickly. Utilities must deal with the fact, however, that regulations presently do not permit the inverters to inject reactive power into the grid.
When equipped with the right controls, battery energy storage systems (BESS) can be used to provide voltage smoothing. Battery charging is not sensitive to voltage intermittency and, once the batteries are charged, they provide a good source of conditioned power. BESS can be located at the substation or distributed along a feeder. An additional benefit is that they can instantaneously provide power to minimize service interruptions. Batteries can provide a firm, dispatchable and renewable resource through smart grid technology.
A more difficult (and less widely appreciated) set of problems is created when a large number of small-scale PV generation sites contribute a significant portion of the power on a distribution feeder. Although some utilities can track the existence of small-scale power producers within their service area, they still do not know how much customer-side resources/storage is available at any given time. This can dramatically influence power quality characteristics on the system by introducing flicker and voltage sags.
Closely monitoring power quality in the distribution system is the first step to solving these problems. Synchrophasor technology that monitors the distribution system by making PMU (phasor measurement unit) readings at high sampling rates -- 30 to 120 samples per second -- is available today. It is also expensive. One can reasonably expect, however, that cost will decrease over time as synchrophasor technology is incorporated into other devices deployed on the grid. In a smart grid context, the ability to detect perturbations almost as quickly as they occur will allow grid operations to take immediate, and sometimes automated, corrective action using either reactive power control or storage.
Transmission System Challenges
For transmission systems, the challenges are roughly the same as the ones just discussed for distribution feeders -- except on a larger scale, yet they are more observable due to the monitoring requirements of the grid in general and at these renewable generation sites in specific. Voltage variability, intermittency and fluctuations are all symptoms of large blocks of renewable energy impacting the system. They can be addressed by correlating renewable energy with the rest of the system load and adding flexibility to the system to correct the problems. System flexibility is the ability of a system to deploy its resources to respond to changes in net load, where net load is defined as the remaining system load not served by the renewable generation.
At the transmission level, a comprehensive approach that includes more than technology deployment is necessary for creating a truly flexible system. An emerging challenge in power system planning is to evaluate the ability of an existing system to successfully integrate (i.e., be flexible enough) the targeted penetration levels of variable generation.
Achieving system flexibility can be accomplished through actions that can include:
- Modifications to conventional power generation resources: These include modifications to ramp rates, minimum up and down time and operating range, including minimum generating level and faster start-up/shut-down times.
- Demand response: Effective demand response programs can provide flexibility over relatively short timeframes when an unpredictable change in variable generation output occurs.
- Variable generation power management (curtailment): Curtailment of variable generation output may be necessary if the amount available at a specific time is more than what the grid can reliably deliver while maintaining reliability.
Other solutions to enable integration of renewable generation include:
- Seeking or demanding greater regional cooperation between balancing authorities.
- Improving compensation and allocation for ancillary services.
- Revising interconnection standards to assign renewables with costs for incremental system security and for ancillary services required to accommodate the operating profiles of their renewables.
- Enhancing wind monitoring, forecasting and communications with grid operators, combined with demand side management.
- Building a new layer of bulk and distributed storage options, which offer greater flexibility than alternatives for meeting sub-hourly dispatch requirements.
When the concept of delivering significant amounts of power to the electricity grid with renewable energy began a couple of decades ago, little consideration was given to how this welcome, but highly variable resource, could be successfully integrated into the existing infrastructure. The time has arrived for the utility industry to come up with definitive plans. Technology is only part of the answer. Relationships must be redefined, standards must be revised and best practices must be developed for planning and implementation.
About the Author
Nicholas Abi-Samra has been actively involved in IEEE for more than 30 years. As Vice President of Asset Management at Quanta Technology, he and his team help utilities better manage and modernize their assets at lower total lifecycle cost. He was both General Chair and overall Technical Program Coordinator for the 2012 IEEE Power & Energy Society General Meeting.