Electric Grid Modeling for Distribution Generation Applications
2012 M.Eng. thesis, McGill University
Written by Michael Nicholas George
Chapter 1:
Introduction
Economic and technological trends have shifted the very concept of the electric power system from tightly controlled, top-heavy unidirectional energy pipelines to systems that are more distributed – both in terms of actual generation capability and in terms of market control. This has posed a huge challenge to the control and protection schemes of these vast networks, most of which are designed and coordinated for conventional power systems with large-scale, centralized generation. With the electric power system becoming more and more decentralized, many researchers and engineers have been working to manage the implementation of distributed energy resources into the existing power grid. Technologies like distributed generation, local energy storage, demand-side response, and the associated means for coordination and communication are being developed in an effort for conventional power systems to evolve into “smart grids” that will handle a variety of multidirectional power flows between many independent parties.
Because distributed generation (DG) sources must comply with dedicated interconnection guidelines, it is necessary for DG impact studies to be conducted in order to assess the effects the DG’s will have on the area electric power system upon connection. The goal of this thesis is to outline a methodology to simplify a utility’s distribution feeder into a representative model that is as simple as possible but with the characteristics preserved that are relevant for power flow studies and transient analysis for events that occur in faults, fault protection actions, and DG response to faults.
1.1 - Distributed Generation
Large-scale generation projects are less likely nowadays to have the political and financial resources to see fruition. The increasing prevalence of environmental concerns associated with CO2 emissions, safety and sustainability of nuclear power, environmental effects of large-scale hydroelectric projects, etc. provide a realm of uncertainty that lingers over any generation company trying to secure capital for a large power plant. In addition, the expansion of transmission networks has been slowed by the financial uncertainties associated with deregulation.
In the meantime, a variety of technologies have focused the spotlight of power systems research and development efforts to local power systems (i.e. distribution level). Distributed generation, energy storage systems, and advanced metering have the potential to give consumers more control over their consumption – and perhaps production – thereby granting a more active role to consumers in the electricity marketplace.
The trend is towards the development of self-reliant local networks that depend less on both the transmission systems and large-scale power producers. It is hoped that this decentralization will not only make power systems more resilient and make electricity markets more competitive, but also defer or eliminate the need for capital-intensive, politically sensitive, large-scale generation and transmission projects in the future.
A conventional power system is composed of three distinct levels of operation – generation, transmission, and distribution. The generation system is responsible for producing electric power from a particular energy source like coal, natural gas, nuclear power, or hydroelectric power. Because these generation facilities are generally very large and environmentally intrusive, they are typically located in remote areas far from urban centers. Transmission lines provide the connection between these large, centralized generation facilities and load centers. Spawning from these transmission lines are distribution networks that handle low-voltage power and connect to loads like factories, businesses, and homes in order to rovide power from the transmission line directly to the consumers as needed. Because of the delicate
balance necessary to constantly match supply to demand, these power systems are highly centralized and
strictly coordinated.
Distributed generation (DG) is a paradigm of electric power systems placing power generation capability at
the distribution level. Individually, these generators are small in power output, compared to those used in
the conventional system. Because of this difference in scale, they do not have the significant negative
environmental impact of larger, conventional power generators; thus, it is possible to place these sources
closer to load centers where they are needed. The small size and modularity of these sources enable
widespread accessibility of potential generation to consumers, who can then sell their own generated
power into a market on the power system. This improved competition gives consumers more choice on
where their power comes from and how much they are willing to pay for it. DG also decreases reliance on
the conventional centralized power sources for electricity, which has the potential to improve access for
remote areas where bulky and costly transmission lines are unable to reach. In addition, because power
near a given DG source is generated locally, less of the power delivered to the load from the DG source is
wasted by transmission losses and other conversion losses. Figure 1-1 illustrates the difference between a
conventional power system and one containing several distributed generators throughout.