As an important form of energy, electricity offers the advantages of being clean, highly efficient and convenient for users. The power system that links the generation, transmission, distribution, and consumption of electricity is one of the most complex man-made systems constructed to date.
With the increasing demand for energy throughout the world and the associated environmental problems in recent years, conventional centralized power systems are facing significant challenges. The development of a highly efficient and environmentally friendly smart grid has become an important objective worldwide.
Smart grid distribution systems will play an important role in future smart energy systems in providing a link between the transmission grid and the consumers.
A smart grid distribution system is the integration of advanced distribution automation, distributed generation and microgrid technologies.
Advanced information, communication, and computation technologies are essential in a smart grid distribution system to support its planning, operation, and control. As a result, the smart grid distribution system becomes a complex cyber-physical system that connects energy and information systems.
Using smart distribution terminal units, a smart grid distribution system is able to ensure optimized operation under normal operating conditions and self-heal when faults occur. It can provide safe, reliable, high-quality, economic and environmentally friendly electrical power.
FIGURE 1: Illustrative diagram of a smart grid distribution system (Click for Enlarge Version)
A typical configuration of a smart grid distribution system is shown in Figure 1.
A distribution system can be divided into high, medium and low voltage levels. For medium and high voltages, the system topology has evolved from a simple radial arrangement into meshed or interconnected systems, in order to ensure high reliability, operational economy, and equipment utilization.
In addition to being connected to the bulk power system, power for a smart grid distribution system is also supplied from distributed generators (DGs) located at the medium and low voltage levels. In this way, the smart grid distribution system is able to take full advantage of renewable energy resources.
Moreover, future smart grid distribution systems will consist of both AC and DC systems, which will better serve a large number of DC loads, such as computers and electric vehicles (EVs).
Characteristics of Smart grid Distribution System
1. Distributed Energy Systems
A major challenge faced by smart grid distribution systems is the diversity in the types of DGs, which include photovoltaic units, wind power systems, fuel cells and micro-turbines, each DG utilizing different energy sources and showing different characteristics.
Due to random fluctuations in some of the resources, for example, wind and solar, energy storage systems (ESSs) are added to ensure the instantaneous and short-term power balance of the entire energy system. These ESSs can be divided into electrochemical types, such as lead-acid, lithium and sodium–Sulphur batteries; mechanical types, such as flywheels and compressed air ESSs; and electrical types, such as supercapacitors and superconducting magnetic ESSs.
Most of these distributed energy resources are connected to the distribution system through power electronic converters, thus providing increased controllability (but reducing the inertia of the power system).
Besides these inputs from distributed energy resources, a large number of new controllable loads, such as EVs and smart home heaters, are emerging. Using the demand-response technology, these controllable loads serve as equivalent (negative) energy sources and participate in the optimal operation of the entire power system.
2. Multi-Layer Autonomous Operation Areas
In a smart grid distribution system, the components can be organized to form autonomous operating areas, in order to manage the DGs and serve the users’ energy demands more effectively. These autonomous areas, such as microgrids, unit control areas (CELLs) and virtual power plants (VPPs), have different scales and operating objectives.
A microgrid is a small, low-voltage power system unit that consists of DGs, ESSs, loads and monitoring and control systems;
A CELL is an extension of a microgrid that covers a larger physical area at a higher voltage level;
And a VPP controls the electricity demand and generation of a large area and by effective management of the controllable loads and DGs replicates the performance of a conventional power plant.
Under normal conditions, these autonomous operating areas satisfy the internal load demand and are operated optimally. In emergency situations, they support each other and maintain power supplies to critical loads. An autonomous distribution area can be dispatched as a whole to facilitate optimized operation at the system level.
3. Information and Communication Systems
A powerful information and communication system is fundamental to a smart grid distribution system to manage tasks such as state awareness, information collection and broadcasting of commands.
Compared with conventional distribution systems, the information and communication system of a smart grid distribution system has evolved considerably into a sophisticated assembly of advanced measurements, two-way, high-speed communication and big data management that stores and analyses information about power distribution and consumption. The results provide data fundamental to the planning and design, optimal operation, simulation and analysis of a smart grid distribution system.
4. Novel Power Electronic Devices
Power electronic converters play an essential role in a smart grid distribution system and bring increased controllability of active and reactive power lows.
The main function of power converters in smart grid distribution systems is to provide interfaces for DGs and ESSs.
Depending on the characteristics and role of a DG or ESS, specific control strategies are designed for their associated power converters, such as constant active/reactive power control, constant voltage/frequency control and droop control. It is usual that these strategies are arranged to perform smooth switching between different operating conditions.
In addition to providing interfaces for different types of DGs and ESSs, power converters can also be used as devices that replace traditional transformers and switches and provide services such as voltage regulation, power flow control, reactive power compensation and harmonic control.
A soft open point (SOP) in a medium- or low-voltage system, which is often based on back-to-back power converters, is a typical example. It is used to replace a conventional switch connecting two feeders and thereby provides a way to optimize the operating condition of the distribution system by controlling the power flow between the two circuits.
In summary, the research and development of power electronic devices have become a critical issue in the development of smart grid distribution systems.