How to Achieve Completely Autonomous Power in the Next Generation of Smart Grids
By Qing-Chang Zhong
The paradigm of future power systems described here offers a method of standardizing the interface of all electrical supplies, including conventional power plants and new add-ons, such as wind/solar farms, electrical vehicles and energy storage systems, and a majority of loads with the transmission and distribution networks, by exploiting the synchronisation principle of synchronous machines. This model opens the prospect of achieving completely autonomous operation of power systems.
Due to civilisation and economic development, demand for electricity is constantly growing, leading directly to supply issues and environmental crisis. Large-scale utilisation of renewables is regarded as a promising means of lessening those problems, and as a result, power systems are going through a paradigm change from centralised generation to distributed generation, and further on to smart grids.
In current power systems, the generation of electricity is dominated by centralised facilities. The lion's share of power is provided in China, for example, by just 1500 or so generators rated at 200 MW and above. It is relatively easy to regulate a limited number of generators in a power system so as to achieve system stability and to meet the balance between generation and demand.
When a large number of new add-ons—wind or solar farms, electrical vehicles and energy storage systems—are integrated into a power system, the number of players on the supply side will increase considerably. Moreover, a lot of players on the demand side are expected to actively take part in the system regulation as well. Hence, the total number of active players in a power system could easily reach millions, hundreds of millions or even billions. How to make sure that all these players are able to work together to maintain the system stability is a great challenge. A simple mechanism is needed to facilitate the organic growth and autonomous operation of power systems.
Adding a communication and information system into power systems would help: hence the birth of smart grids, power systems with communication and information systems added to operate in parallel. With the introduction of smart power, systems will become more efficient and more resilient in the face of threats, and friendlier to the environment. Naturally, the added communication systems are expected to provide the infrastructure needed for all power system players to work together, even at the low-level controls. This standard scenario, however, brings with it serious concerns about reliability. If the communication system breaks down then the whole power system could crash. Moreover, when the number of players reaches a certain level, how to manage the communication system is itself a challenge.
A vision of next-generation smart grids I have devised would allow all active players to communicate with each other at the bottom level of the power system, without relying on a communication network. The function of communication is achieved through control, that is to say, the measurement of local voltage or frequency and the execution of control algorithms, based on the underlying synchronisation mechanism of synchronous machines. As a result, the communication system in a smart grid can be released from low-level controls and adopted to focus on high-level functions.
In such a power system, all conventional power plants, including coal-fired, hydro and nuclear, are connected to the transmission and distribution network through synchronous generators, as normally done.
With respect to new sources of generation, the various different types of renewables, electric vehicles and energy storage systems can all be connected to the transmission and distribution network through DC/AC converters, also called inverters. These inverters can be controlled to have the synchronisation mechanism of conventional synchronous machines, by means of the self-synchronised synchronverter technology that is further developed from the synchronverter technology I invented with George Weiss, Professor of Control Engineering at Tel Aviv University, Israel. More specifically, the mathematical model of conventional synchronous generators can be taken as the core of the controllers for these inverters, and the mature technologies developed for conventional synchronous generators can be adopted and wrapped onto the core of the controllers. Hence, these inverters have the dynamic behaviour of conventional synchronous generators, in particular, the synchronisation mechanism.
As for loads, to a great extent they also can be integrated and controlled by means of synchroverter technology. In the United States, according to a report from the Electrical Power Research Institute (EPRI), electric motors account for about 50 percent of end consumption, lighting 20 percent and the Internet about 10 percent, with miscellaneous items making up the remaining 20 percent. In order to improve the efficiency of motor systems, it is very common to use variable speed drives, which include AC/DC converters (also called rectifiers) at the front end to interface with the grid. Internet devices are powered by DC supplies, so they interface with the grid by means of rectifiers as well. For lighting devices, it is already a clear trend that LED devices will dominate the market in the near future. LED lights are powered by DC supplies as well.
So, the majority of electricity in a future power system—probably 80 percent or more—could be powered via rectifiers, regardless of the different functions of loads. These rectifiers can also be controlled to have the synchronisation mechanism of conventional synchronous machines, using the self-synchronised synchronverter technology. More specifically, they can be controlled to have the dynamic behaviour of conventional synchronous motors. Because of the availability of the frequency information inside the rectifiers, it is very easy to implement continuous rather than on/off demand response, which provides a means to fully release the power of demand response.
It is well known that synchronous machines can synchronise with each other or with the power supply autonomously, without the need of external communication. The above framework for next-generation smart grids simply turns all supplies—both conventional and new add-ons—and the majority of loads into synchronous machines. They can work together autonomously and make equal contributions to maintain system stability, so as to achieve completely autonomous power systems.
Qing-Chang Zhong, a senior member of IEEE and a fellow of the Institution of Engineering and Technology (IET), is Chair Professor in the Department of Automatic Control and Systems Engineering, University of Sheffield, UK. Jointly with George Weiss, a professor of control engineering at Tel Aviv University, Israel, he invented the synchronverter technology to operate inverters to mimic synchronous generators, which was recognized as “highly commended” in the 2009 IET Innovation Awards. He received a Ph.D. degree in control and power engineering (awarded the Best Doctoral Thesis Prize) from Imperial College London in 2004, and a Ph.D. degree in control theory and engineering from Shanghai Jiao Tong University in 2000, and is the co-author of three research monographs, including Control of Power Inverters in Renewable Energy and Smart Grid Integration; a fourth, Completely Autonomous Power Systems (CAPS): Next Generation Smart Grids, is scheduled to appear in 2015. In 2012-2013, he spent a six-month sabbatical at the Cymer Center for Control Systems and Dynamics (CCSD), University of California, San Diego, and an eight-month sabbatical at the Center for Power Electronics Systems (CPES), Virginia Tech, Blacksburg. Va.