Improvements in Grid Connectivity to Bring Renewables to Where the Supply Is
By Hibba Ahmed
How does the power grid work?
Electricity is a vital resource that everyone uses at some point in their daily lives. A power grid is a means through which electricity can be delivered from producers to consumers.
This image displays the major components of a power grid.
The first step for energy on the electric grid is power generation, where electricity is produced. The power plants can take one kind of energy and then convert it into electrical energy. Since power plants are located far away from populated areas, the electricity needs to be efficiently transported. This efficient transportation is handled by high-voltage transmission lines. Before the energy is transmitted, transformers at the plant increase, or step up, voltage to reduce losses within the lines as the electricity travels to its designated location. Once the electricity arrives at populated areas, transformers decrease, or step down, the power to a safer voltage. This process occurs at a substation. During distribution, some customers get power directly from transmission lines, but many are served from feeder lines that carry power from the substation. From the feeders, smaller transformers step down voltage to its final level for industrial, commercial, or residential use before the electricity reaches its destination.
Why is voltage stepped up only to be stepped down?
Electrical power consists of both voltage and current so that power = voltage x current. Voltage is first stepped up by a transformer and then the power travels through transformer lines to a neighborhood. Before entering the neighborhood, the voltage is stepped down by yet another transformer. This process occurs because the goal is to provide the neighborhood with as much power as possible. However, power is lost in the journey because as power travels through the lines, friction turns electrical energy into heat energy which dissipates into the air. Since power loss = (current)2 x resistance, it can be seen that power loss increases exponentially with current increase. Thus, it is important that the current is kept as low as possible while not reducing the amount of power that is transmitted. Therefore, if voltage is increased by an amount proportional to the decrease in current, then power is kept at the same level. Increased voltage allows a decreased current which reduces how much power is lost. Once the power completes its journey we decrease its voltage at a step-down transformer to make it safer and usable in the neighborhood. Lower current also enables power to be transmitted over thinner electric lines which are less expensive than thicker ones. Thus, minimizing the current saves money by minimizing both power loss and construction costs.
Many power grids use alternating current, or AC, where the direction of voltage and current are constantly switching, in contrast to direct current, or DC, in which there is a constant flow of current in a single direction. AC power is useful because it is easy to increase or decrease the voltage, which is essential in safely moving electricity from the producer to consumer.
What do we look for in efficient power grids?
The three main goals that producers wish to achieve when creating an efficient power grid are power quality, reliability, and market equilibrium. Electrical devices and equipment are designed with the assumption that the power coming from the grid has certain parameters, primarily that the voltage and frequency are accurate and stable. Changes in the voltage can lead to brownouts, where there is a reduction in the availability of electrical power, or surges, where there is a spike in the electrical system’s current, that can damage connected equipment. The reliability of efficient power grids can be accounted for by the constant availability of power as well as the ability of electricity to be rerouted when a piece of equipment is out of service. Lastly, market equilibrium, or the point at which supply meets demand, is crucial in constructing a grid that can efficiently transport electricity from one place to another. Millions of people connected to the same grid smooths out the demands created by individuals, but load following, which is a dispatch strategy that produces only enough power to meet the primary load, is still a big problem. The demand for electricity usually follows a consistent pattern, but factors like extreme weather can make it difficult to predict. Thus, grids are made smarter by using software, sensors, and devices that can communicate with one another. On the supply side, smart grids provide computers and software with data to help companies in making decisions about how to manage the grid. Smart grids can also help on the demand side. Since consumers usually do not have a proper understanding of how much power they use, or how much energy should cost depending on the time of day or year, a smart grid can clear some of the confusion. Therefore, smart grids can enable consumers to make better decisions in respect to how they use electricity in their daily lives. Ultimately, smart grids can allow producers to take care of the power grid more efficiently and effectively.
How do renewables connect to the grid?
Producing electricity using renewable energy resources, such as solar and wind, rather than fossil fuels, such as coal, oil, and natural gas, cuts down greenhouse gas emissions and aids in addressing climate change. Although renewables are beneficial to the planet in terms of emissions, renewable sources are dependent upon variable natural resources. Variable natural resources are generation resources whose output cannot be perfectly controlled by a transmission system operator, which transports energy using fixed infrastructure, and whose output depends on a fuel resource that cannot be directly stored as well as whose availability is sometimes difficult to predict. Wind and solar power generation are the primary examples, since the sun does not shine all the time and the wind does not blow all the time. Even during the day, clouds and dust can interfere with solar power generation. Thus, these plants are harder to control and challenging for grid operators.
In order to balance supply and demand of electricity on the power grid, operators must have an idea of how much renewable energy is being generated at any moment, how much is expected, and how to respond to changing generation. This information can be difficult to obtain because of how intermittent renewable power can be in addition to the various sizes and locations of renewable energy resources across the power grid. As mentioned above, variable renewable energy (VRE), or intermittent renewable energy sources (IRES), are renewable energy sources that are not dispatchable, or able to generate power when demanded, due to their fluctuating nature, such as wind and solar power, as opposed to controllable renewable energy sources, such as dammed hydroelectricity or biomass.
The two main types of renewable energy generation resources are distributed generation, which refers to small-scale renewables on the distribution grid where electricity load is served, and utility-scale generation, which refers to larger projects that connect to the grid through transmission lines. In contrast to centralized renewable plants that connect to the grid through high-voltage transmission lines, distributed resources are connected through electrical lines on the lower voltage distribution network. These projects often occur “behind the meter,” which means that energy is generated for on-site use, such as rooftop solar panels that provide a household with power. These distributed projects usually lower the demand for electricity at the source as opposed to increasing the supply of power on the grid. Community-scale renewables reside “in front of the meter,” because the power they generate is not used on-site but flows onto the distribution grid to be used by homes and businesses in the area. Centralized projects are easier for the grid operators to control because it is easy to track how much electricity is demanded. Since distributed renewables are usually small and behind the meter, they can be difficult to track and can complicate load forecasting, which is a technique used by companies to predict the amount of power/energy needed to meet the supply and demand equilibrium. Nonetheless, distributed renewables can still provide the grid with benefits that large projects cannot. Since the energy from distributed generation is usually used nearby, distributed energy resources can reduce energy losses that occur when electricity is carried on transmission lines.
Since weather can change quickly and unpredictably, high renewables penetration requires grid operators to be flexible and quickly react to new conditions and production patterns. Even if the weather is predictable, grid operators have to quickly respond to falling production from solar energy when the sun goes down but the demand for electricity stays the same.
This curve is called the “duck curve” because the difference between the actual and net load looks like the body of a duck. This curve demonstrates the difficulties of controlling a system with a high penetration of renewables. In an attempt to find a solution to this problem, grid operators will sometimes curtail output from certain renewables during times of high production if there is not enough demand. Counterintuitively, this can result in economic losses for renewable generators and a loss of clean energy production. Thus, other options will be explored in this research paper in an effort to increase grid flexibility and improve the integration of renewable resources.
How can the grid be improved to bring renewables to the supply?
As more VRE is integrated into the electric grid, problems arise from the need to maintain the balance between load and generation. The primary attribute of VRE that must be addressed is the variability of the resource and how to account for this variability over a range of timescales. Since VRE is not dispatchable, there are many opportunities to construct grids that are more flexible and can integrate larger amounts of VRE.
If distributed renewables are connected to microgrids, they can provide increased resilience during storms or other natural disasters by providing power even if the larger grid experiences outages. A microgrid is a local energy grid with control capability, which means that it can disconnect from the traditional grid and operate autonomously by using local energy generation. A microgrid connects to the grid at a point of common coupling that maintains voltage at the same level as the main grid unless there is a problem that causes the grid to disconnect and function separately as an island. Microgrids not only serve as a backup for the grid in emergencies, but they can also cut costs and connect to a local resource that is too small or unreliable for traditional grid use. Thus, microgrids can strengthen grid resilience, decrease power outages, and provide energy resources for faster system response and recovery.
Additionally, energy storage can be coupled with variable renewables to account for fluctuations in renewable generation. Electricity can be stored during times of high generation, for solar during sunny days and for wind during times of high wind speeds, and for later use during times of high demand. The electricity can be converted to different forms of energy for storage and then converted back to electricity on demand. Therefore, energy storage can be used for a variety of functions including regulation, load following, and energy shifting to add or absorb energy from a power system when there is a surplus or excess of renewable energy.
Furthermore, variable resources that come from many geographically diverse areas have shown a decline in the system-wide variability for both wind and solar power. This can only be achieved as the interconnected system size increases even though some smaller systems may not have any way to increase the geographic diversity of their system. Thus, geographic diversity plays a crucial part in ensuring that energy can be efficiently delivered to the consumer at the proper time.
Conclusion
Renewable energy resources provide huge potential for a low carbon future, but their intermittent nature on the electric grid has to be taken into consideration. This paper overviewed the fundamentals of the power grid and its relation to renewables in addition to discussing several solutions that could aid in increasing the flexibility and resiliency of the grid with higher levels of VRE penetration. In the future, a modernized grid will serve as an integral part in ensuring a successful transition to an electrical grid that is dominated by renewables.