International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 www.as‐se.org/ijpres The Impact of Renewable Energy Sources on Utilities Ziyad Salameh, Life time Senior Member IEEE Department of Electrical and Computer Engineering, University of Massachusetts Lowell, Lowell, MA, USA Ziyad_salameh@uml.edu Abstract Recently the global warming, pollution and high oil prices forced politicians, utility companies (UC) and the general public to pay more attention to renewable energy sources (RES) such as wind, photovoltaic and bio fuels. RES may be located on backbone of the distribution systems; they are located right where the customers are, so they are used more efficiently, they are not polluting, renewable and modular. The application of RES has an impact on the utility companies, manifested in: The potential reduction in the demand side load which is equivalent to increasing the capacity of the utility companies, the reduction in summer peaks load from the application of PV systems. Because of the intermittent nature of the RES, their penetration level in a UC is important to be figured out in light of the energy storage available to the utility. Also the UC have to worry about the interaction with customers who used this RES, which includes: Protect the safety and integrity of the UC system, prevent islanding, and the contribution to short circuit current in the utility system, the metering policies, the cost of interconnection with the qualified customer, the power quality. The overhead lines needed to carry the RES from the abundant sources of renewable energy to the customers is an important factor in their wide spread use. Also legislative issues: The zoning policy of installing small scale RES in towns and cities has to be dealt with and legislated, the impact of incentives given by governments to customers to encourage the use of RES and as a result of that the complexity of planning and managing the UC. Finally the UC need an abundant and skilled workforce to design, build, operate and maintain RES. Keywords Renewable energy; Wind; Photovoltaic Introduction
Constantly growing demand for energy cannot continue indefinitely relying only upon fossil fuel. The earth’s finite supply will eventually exhaust. Energy is a major key to industrial development and the world’s well‐being .The awareness of depletion of fossil fuel resources has challenged scientists and engineers to search for alternative energy sources that can meet energy demand for the near future. It has been recognized that attention should be paid to resources that are continuous, free in their availability, and pollution free [1]. Renewable energy sources (RES) such as wind and photovoltaic are now have passed the experimental stage and are widely used all over the world. According to their installation RES can be divided into stand‐ alone generating systems, which are also called off‐ grid systems and grid‐connected systems, the later present challenges to the utility companies. According to the U.S. Energy Information Administration (EIA), in 2012, 37% of all U.S. electricity was generated from coal fired power plants. Of all fossil fuel sources, combustion of coal releases the greatest amount of Carbon Dioxide per British Thermal Unity (BTU) of energy. The EIA monthly energy reports for September 2013 project world power sector CO2 emissions reference case on an upward trajectory towards 58 Giga tons of CO2, of which the US contributes 5.4 GT, and the power sector itself contributes 21%. This is projected to expand to 5.65 GT/Yr. in 2040. In 2012, almost 40% of total energy related CO2 emissions in the US resulted from electricity generation. About 75% of those overall power sector emissions resulted from burning coal. Despite a trend to natural gas use which results in a 60% reduction in GHG loading potential per unit energy produced, upward trajectories of worldwide carbon emissions are projected out to 2040. This unabated emissions scenario is associated with various climate model outputs estimating a long term global temperature rise averaging 6 degrees Celsius, which, according to many studies would result in significant changes to earth’s climate, oceans and landmass. Significant emissions reductions in excess of 50% from a 2009 baseline of 31GT, would be necessary to limit modeled global temperature increases to 2‐4 degrees Celsius. According to IEC, renewable energy in 2009 73 www.as‐se.org/ijpres International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 accounted for 3,902 TWh and provided only 19.46% of worldwide electricity generation. Of this amount, wind and solar accounted for only 15%. To meet our climate and energy goals, significant build out of low renewable sources is required, and should be expected. The vast majority of installed renewable energy resources are designed to deploy power to the electricity grid. While many sources including Photovoltaic (PV), wind power, geothermal, hydro, biomass, and combined heat and power (CHP) may be used to support direct current (DC) applications and individual users loads “off the grid”, many more are focused on deployment as renewable distributed generation sources (RDGS). Many sources indicate utility scale wind power is the most cost effective and scalable renewable resource to expediently drive renewable power onto the grid today. At the end of 2013, global installed wind capacity exceeded 318 GW, of which more than 60 GW was installed capacity in the U.S. The issues of reliability and intermittency of this resource, along with difficult forecasting, centralized sitting of wind farms which are sited in identical weather micro climates, and typical areas of installation being distance from transmission lines is similarly well documented. The primary issues associated with wind and PV resource deployment are caused by its reliability and generation intermittency. Those issues result in difficulties with local grid load/demand leveling, power output and energy quality control with regard to voltage, frequency, and total harmonic distortion. These issues appear to be compounded in distribution grids that contain a high penetration of renewable resources. While many reports and studies are still reviewing impacts downstream of these facilities, many studies are suggesting local penetration rates of 5, 10 and 20% are trigger points where additional study or actual hardening actions should be implemented at local grid infrastructure to control voltage and other types of output fluctuations that could negatively affect overall grid power quality. Impacts of RES on Utilities
Help the UC Meet the Load Demand Demand side load reductions during times of system’s need, such as periods of peak demand or high market prices. Because reduced consumption and increased generation can both restore a systemʹs supply and demand to equilibrium, RES can be resources that offset or defer the need for new generation, transmission, and/or distribution infrastructure. Drawback on RES is restricted control on the time and demand of utilizing, which could be improved by administered load management programs. For instance, planning for seasonal and daily period leads to better implementation of policies by utility companies. In this regard, solar energy can act as demand response resources specifically during summer peak load. On the other hand, demand response programs in a positive feedback can provide flexibility that enables the electric power system to more easily integrate wind and solar—and may be cheaper than alternatives. Intermittency and unpredictability nature of solar energy would change in summer and consequently it would be a reliable solution for demand reduction during day hours. The impact of PV’s on substation peak load is highly dependent on the time of substation peak and it would vary from region to other. FIG. 1 HOUR PV OUTPUT (BOSTON) AND ISONE ANNUAL AVERAGE HOURLY LOAD CURVE (2011) On the other hand, For the most part, when electricity demand is highest (in California), the sun is shining brightly. An important advantage of PV generation is that maximum PV output is generally during peak load demand. Figure 1[2] compares the summer average load curve for a Boston area with the summer average PV output curve. There may be opportunities for PV and other demand side measures to contribute to point of use energy supply, 74 International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 www.as‐se.org/ijpres thereby reducing the need for central generating plant and for costly network upgrades. PV output peak hours largely coincide with peak electrical Demand periods both during the 24‐hour period of the day as well as summer peak months. Summer peaks are significantly higher than winter peaks both throughout the US. The wind energy peaks in winter and a hybrid wind/PV the two systems complement each other figure 2 [3]. FIG. 2 HYBRID WIND/PV OUTPUT AT UML Metering Problem In 1978 the Congress passes PURPA (public Utility Policies Regulatory Act). PURPA requires electric utilities to purchase electric power from and sell electric power to cogeneration and small power production facilities. PURPA requires electric utilities to purchase electric power from and sell electric power to cogeneration and small power production facilities at the avoided cost which is defined as the costs to an electric utility would have had to generate or construct itself or purchase from another source. 1)
Net Energy Metering Net energy metering, or ʺNEMʺ, is a special billing arrangement that provides credit to customers with solar PV systems for the full retail value of the electricity their system generates. Under NEM, the customerʹs electric meter keeps track of how much electricity is consumed by the customer and how much excess electricity is generated by the system and sent back into the electric utility grid. Over a 12‐month period, the customer has to pay only for the net amount of electricity used from the utility over‐and‐above the amount of electricity generated by their solar system (in addition to monthly customer transmission, distribution, and meter service charges they incur) Figure 3. Because Solar customers do not pay their full share of the Utility infrastructure costs, more and more selling electricity will cut on the profit of the utilities. That leads to high rates, although it encourages the use of RES. FIG. 3 DIAGRAM OF GRID CONNECTED PV SYSTEM WITH NET METERING 75 www.as‐se.org/ijpres International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 Non-Controllable Variability, and Partial Unpredictability of RES
Renewables are intermittent in nature. The only way to maintain grid reliability and stability is to have solid base load like nuclear and significant dispatch able resources like gas‐fired turbines. These sources must be able to ramp up and ramp down RES in a manageable and predictable fashion [7]. Solar and wind are indeed variable, ways to balance variable renewable, including geographic dispersion, which has a strong smoothing effect on variability, and base load renewable like biomass, geothermal and hydro power also back up traditional power sources like large coal, nuclear and natural gas plants When RES generate redundant power that would already be generated by base load generation plants, for that sufficient grid level storage should be available to store it. It is clear, even with wind and solar PV, base load power is still needed to provide power at times when wind and solar PV donʹt provide power. So, without grid level storage which is currently too expensive, excessive amounts of wind and solar PV power are not beneficial. The availability of wind and sunlight is partially unpredictable. A wind turbine may only produce electricity when the wind is blowing, and solar PV systems require the presence of sunlight in order to operate. Actual wind power can differ from forecasts, even when multiple forecast scenarios are considered. Unpredictability can be managed through improved weather and generation forecasting technologies. Interconnection Problems
Islanding The condition in which a distributed generator (DG), continues to power a location even though electrical grid power from the electric utility source is no longer present, is called Islanding. If an island forms, utility repair crews may be faced with unexpected live wires. All solar installations must include a safety trip that causes the solar system to shut down when the grid shuts down. The DG should be disconnected from the grid, and forced to power the local circuit or just disconnected. Anti‐islanding protection can be improved through the use of the Supervisory Control and Data Acquisition (SCADA) systems already widely used in the utility market. Potential Contribution of the RES Inverters to Short‐Circuit Current Levels Inverters have introduced new challenges for utility engineers to determine changes in the short‐circuit capacity of a system and potential impact on interruption rating of switching devices. The issue is more complex in nature when several renewable generation sources are connected to the same system (e.g., a distribution feeder or a circuit) as the aggregated effect of the contributing sources to the fault and interaction among the electronically controlled generation units are unknown. It is to be noted that advanced inverters are capable of limiting the short circuit current to 150% of the full load. A study by Farid Katiraei [4] showed that the PV inverter current contribution during a fault is not zero and varies by design. Also it was observed that, for most fault conditions, several PV inverters continued supplying current to the feeder, subsequent to a fault for a period ranging from four to 10 cycles. Moreover the current contribution level is a function of the voltage at the terminal of the PV inverter during a fault, which is determined by the type and location of a fault. Harmonics Harmonic distortion is known as a serious power quality problem, which may occur due to the use of power inverters for converting DC current to AC current in RES systems. The produced harmonics can cause parallel and series resonances, overheating in capacitor banks and transformers and false operation of protection devices which may decrease the reliability of power systems. According to IEEE standard 519 total Harmonic Distortion (THD) should be less than 5% [1], also no individual Harmonic content should exceed 3% of the fundamental. In other hand, when multiple distributed generation units are operating at different points of common coupling on the same feeder, each distributed generator may meet the IEEE Standard 1547 current injection limit; however, the aggregate impact of the units could cause voltage distortion that would adversely impact other customers. 76 International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 www.as‐se.org/ijpres Inrush Current The small inevitable difference between the RES inverter system and the grid voltages may introduce an inrush current which flows between the PV system and the utility grid at the connection time and decays to zero at an exponential rate. The produced inrush current may cause nuisance trips, thermal over stresses, and other problems. Whether Related Problems
RES output depends on resources that are location dependent, how windy and sunny the location is, unlike coal, gas, oil or uranium, cannot be transported to a generation site that is grid‐optimal. Generation must be collocated with the resource itself, and often these locations are far from the places where the power will ultimately be used. New transmission capacity is often required to connect wind and solar resources to the rest of the grid. RES when dispersed over wide geographic areas their generation can fall well below nameplate as a consequence of weather and the daily rotation and seasonal tilt of the planet. Making weather dependent generators, such as wind and solar or a combination thereof, a form of dispatch able capacity is quite a bit more involved and costly. A passing cloud removes and dumps large PV power on the utility. Also the variable nature of wind speed, a wind storm (gust) also dumps large power on utilities. To smooth the variable output of the RES caused by weather change, storage is needed. EPRI reported that the benefits of energy storage in almost every use case studied outweigh the costs. Legislative Issues
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The zoning policy of installing small scale RES in towns and cities. •
Cost of interconnection with the qualified customer has to be paid by the customer •
Standards (Such as Noise Level) Generated by the wind turbine and the Inverter. •
Acceptable power factor. Reduced PF due to switching in large active power, due to weather related conditions. Customers with PF <0.85 should pay penalty. Large Scale Application of RES
Renewable generation types – wind, solar, and wave – that are subject to natural variability in their energy sources. This variability creates distinct challenges for integration into the larger power system, namely non dispatch ability. Secondly, wind and solar are relatively mature for use in large capacities and in wide areas, and so have a significant impact on the power grid that is likely to increase over time. The use of large scale clusters of RES demands the coordination of using RES, transmission lines and energy storage resources. Integration of large‐capacity RE sources and the application of large‐capacity energy storage for that purpose is a worldwide direction [8]. RES are a growing component of electricity grids around the world due to its contributions to (1) Energy system decarburizations (2) Long term energy security (3) Expansion of energy access to new energy consumers in the developing world. High Penetration level of RES causes Difficulty in planning, control and operation of Utilities
As the portion of electrical energy produced by distributed RES increases, concerns heighten over the potential for such resources to create steady‐state voltage or current violations on electrical distribution feeders. That could cause instability [9]. California utilities and German utilities are facing significant penetration of distributed generation assets, mostly 77 www.as‐se.org/ijpres International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 rooftop solar, on their existing operations. But utilities are going to have to cover the costs of maintaining the grid in any case, and without changes, that means that solar‐equipped customers, who tend to be wealthier than not, may be pushing extra costs onto ratepayers that don’t have solar. Electric Power Research Institute (EPRI) noted that utilities don’t just provide backup power to self‐generating customers. They also provide the “start‐up” power, in volumes or qualities needed to start up heavy loads like air conditioners or refrigerators in homes, or big industrial motors in industrial settings. That has a great value. As soon as utilities start to see a significant erosion of their revenues and the potential for stranded assets, the financial community will get involved. Conclusion
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Renewable energies, driven by global warming, fuel security, and pollution, will be providing more and more of our electricity in the future. They represent an opportunity and a risk. •
RES integration challenges are Numerous‐In‐Principal –Manageable. Accurate modeling, simulation and analysis tools are necessary for studying power systems to derive adaptation strategies •
Increasing amounts of interconnected energy storage have smoothing effect against the variation in output of RES, increase reliability and use of RES. Storage ramps up and ramps down the RES. •
Advanced Inverter technology is the key technology to have reliable and safety grid interconnection operation that communicates their status, generates reactive power and limits short circuit current, multi inverter strategy is to be adopted instead of using big inverter. •
Utility operators in the future will have access to improved forecasting methods and models •
The use of smart Grid, micro grids, super grids will help integrating RES to the utilities. The Smart Grid of the future will manage the ambiguities of load balancing seamlessly. Many of the challenges integrating renewable simply stem from slow communication and lack of effective real time control over varying loads and generation capacity. The power grid actively seeks to balance load (demand) and generation (supply) on the order of fractions of a second. To do this, grid operators need real time insight into time and magnitude changes in both supply and demand REFERENCES
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uncertain‐edge‐of‐the‐renewable‐powered‐grid Ziyad M. Salameh received his Diploma from Moscow Power Engineering Institute in 1974 and his M.S and Ph. D from University of Michigan, Ann Arbor, in 1980 and 1982 respectively. He is currently a professor at the University of Massachusetts, Lowell. He is also Director of the Center for Electric Cars and Energy Conversion. His areas of interest are electric vehicles and renewable energy sources. He has authored or co‐authored over 120 research papers. Professor Salameh is the recipient the IEEE PES Ramakumar Family Renewable Energy Excellence Award for 2015. 79