What Can Distribution Planning Learn from Transmission?
- Written by Hugo Bashualdo, Brian Gemmell
The challenges to distribution planning in harnessing some smart grid technologies are described at a high level, with observations about the potential impacts and lessons that distribution utility executives and planning engineers can expect in the years ahead. In effect, distribution planning faces complex analytical challenges of a kind long familiar to transmission planners.
In the late 19th century, Nicola Tesla won the AC versus DC debate and established the basic principles for future power systems. The power industry over the past hundred-plus years has experienced unprecedented growth due to technology advancements and engineering principles. Electricity is fundamental to modern society, and planning grids to meet load growth remains challenging.
Today, economic and environmental concerns are driving government policy and distribution utility executives to support the use of new renewable generation technologies, load management schemes and other smart grid technologies. The impacts of these policies and technologies are far reaching and touch all aspects of the network from equipment ratings, to protection and control schemes, tariff structures and business process. Because of these ramifications, distribution planning is now entering a new era.
Traditionally, distribution planning has been load serving and more concerned with ensuring the required installed capacity, with the fundamental assumption that power flows in one direction from the source towards the load. Transmission planning, in contrast harnesses multiple sources of generation with loads much more geographically widespread; the assumption in transmission is that power will flow in different directions, following the path of least resistance.
At the same time, the reliability of multiple transmission circuits must be ensured for all credible contingencies to maintain continuity of supply and avoid any brown/black-outs. Thus, over the past forty years, transmission planning has emphasized early adoption of new technologies in the form of HVDC transmission, FACTS and special protection schemes (SPS) in order to comply with reliability standards and grid codes.
Distribution planning now faces complex analytical challenges of a kind long familiar to transmission planners. With the advent of new smart grid technologies, power will flow in different directions, driven by generation and storage technologies connected in the medium and low voltage networks. These new technologies include distributed generation, energy storage, electric vehicles, microgrids, distributed automation and demand response.
The generation output from wind and solar energy is of course highly intermittent, which complicates the network analysis energy-balance task. In addition, the electrical characteristics of renewable generation technologies are undergoing rapid development and will likely be different in the future. The successful integration of distributed generation relies heavily on effective planning and operation. Planners need to consider probabilistic steady-state and dynamics, and also explore loadings, constraints, reliability, voltage control, power quality, fault ride through and short circuit conditions. Attention to optimized siting, sizing to minimize losses and proper protection coordination is also critical.
Technologies are increasingly within reach to support large-scale bulk storage and community energy storage. Devices such as the flow battery, liquid metal battery, sodium-sulfur battery, vanadium redox battery and hybrid flow battery have drastically improved in terms of fast response, charge/discharge performance and transportability. Thus, energy storage is quickly becoming a tool that is accepted as an important component of the modern electrical grid. To employ storage to best effect, topics in need of further study include new capacity deferral by peak shaving assessment, integration of intermittent renewable generation, management of congestion, reliability improvement and planned islanding schemes, integration of electric vehicles and energy management for microgrids.
In many places, governments are pushing to substitute combustion engine cars with electric vehicles. The existing medium and low-voltage networks will be affected, as the typical household load will increase, likely causing overloading of network components. Utilities need to identify the optimal location for home chargers or fast-charging stations, as well as define the necessary network extension and charging strategies in order to optimize network operations by leveraging communications and intelligent control.
Traditional commercial and industrial load can integrate generation schemes into their system, creating a microgrid that is a combination of on-site generation, load, storage, monitoring and control systems capable of operating in both grid dependant and grid independent modes. Likely studies for microgrids will be reliability improvements, cost evaluation of renewable generation/storage target achievement, optimization of generation and losses, evaluation with participation in demand response programs, as well as monitoring and control technologies.
Distributed automation or self healing schemes will enable real-time adjustment to changing loads and generation, as well as distribution system failures with or without operator intervention. This will depend on critical information, communication and proper settings of the field devices. Automating distribution feeders and substations will improve reliability and increase operation efficiency. Fault location, isolation and system restoration (FLIRS) capability enables distribution utilities to meet reliability targets. The challenge is to identify optimal sectionalization and automation deployment with an economical evaluation and cost-benefit analysis.