Linash Kunjumuhammed - Simulation of Power System with Renewables

Chapter One

Frequency; Phasors; Power system; Pu conversion; Stability; Steady state

Electric power system has a fascinating history of evolution over 130 years. Such a long and successful legacy can be described through four distinct phases:

Viable (18801900) : Following the invention of carbon filament lamps in 1879 by Thomas Alva Edison, Pearl Street station in New York was illuminated with a small DC system ( square mile area) in 1882. Many installations followed in the early and mid-1880s in the United States and continental Europe. It had two voltages (100 and 110 V) that reduced operating efficiency of the network as current flow over longer cables had losses in the conductor and drop in voltage at the load. Thus, it limited the size and spread of the network. Around this time, based on the principle of Faraday's law, a device was developed in Europe, which could transform voltage. With financial support from George Westinghouse, William Stanley worked on this device further to develop what we know as proper transformer. Inspired by this success, Westinghouse installed a commercial AC system in Pennsylvania in 1886 on a trial basis, and by 1887, more than 30 Westinghouse AC systems were in operation. Nicola Tesla invented three phase magnetic field and induction motors in 188687. George Westinghouse bought all his patents on AC generator, motor, transformer, transmission and distribution system and employed to further developing the products from these concepts. This breakthrough provided a full-scale industrial war between ACDC. The battle between ACDC continued for several years until AC system had the final triumph when 1893 World's Fair in Chicago was illuminated by Westinghouse with a dozen of 750 KW AC two-phase 60 Hz generators supplying 8000 arc lights and 130,000 incandescent lamps. The electrification arrangement in the fair turned the buildings into a city of light. Later on, more AC systems were built with synchronous generator driven by steam and hydroturbine.

Scalable (190050) : Usually the source of generation being coal or other fossil fuel, it was realized that to make electricity affordable, it must be generated in bulk. So larger size generators were designed and made. But the site source of the fuel. Large synchronous generators are used to generate power at a voltage up to 25 kV. This is stepped up to transmission level voltage, which depends on the adopted standard value transmission voltage of a country, distance between the generation and demand, etc. Typical standard transmission voltages around the world are 220, 275, 345, 400, 500 and 765 kV. When the distance is far too long (about 1000 km or more), high-voltage DC transmission (HVDC) is used. HVDC is also used to connect to AC system of same or different operating frequency. At the consumers' end, the voltage is stepped down to subtransmission and distribution level at stages through step-down transformers. Usually, the consumer voltage is 400/415 V (line to line) or 110 V (mainly in the United States).

Figure 1.1 AC interconnected power system.

With such extra high voltage of transmission, the capacity and spread of the system grew enormously to connect almost every home and factory in the OECD countries. According to the survey of US National Academy of Engineering in 2000, North American electricity system was ranked the number one engineering achievement in 20th century as it is the largest man-made machine on the earth.

Reliable (195075) : A 1000 MW power generator when connected to a network of hundreds of mile long 500 kV system through equivalent capacity transformers in power substations is subject to failure or shut down because of natural causes such as lightning stroke on the them, other operational failures such as certain quantities (e.g., temperature) exceeding design limits and so on. So their round-the-clock availability became very important. Interconnected power network of any reasonably sized country will have hundreds of such generators, transformers and thousands of km transmission circuits, and failure of any one of them should not lead to loss of supply to the customer. There have been several instances of system collapses (1965 power blackouts in the United States and many more after that throughout the world), so the reliability of the system became very important. Many countries have set up reliability council or group to come with operating practices and supply reliability standards, which are known as grid design and operation code. Although probability of failure of equipment can never be eliminated, how to maintain the demand despite outage of certain components was the direction of engineering innovations such as protection and relaying of the system. Improved design standard evolved.

Sustainable (1990-present) : 66% of electricity generation in 2016 in OECD countries was from fossil fuel. When including non-OECD, this figure will go up further. For about 25 years, this percentage was even higher. Given that the gas reserve is predicted to last for another 50 years and the world will run out of coal in another 150 years with present rate of consumption, the source of energy has to move toward renewable form. The other important concern is the contribution of electricity generation into greenhouse gas emission. In 2015, it was estimated that in the United States, 29% of total greenhouse gas emission was contributed from its electricity generation. This figure for other large developing countries is even higher. So from the perspective of reduction in greenhouse gas emission, generation from low-carbon technology for the sustainability of the world economy and environment is important. There has been a massive growth of generation from renewables. In 2017 alone, 52 GW of wind and 95 GW of solar are added, taking the total installed capacity to 400 and 540 GW, respectively (WEC). Several countries have set targets of increasing percentage of generation from renewables. Already several countries in Europe such as Germany have several days at a stretch of electricity demand met from renewables only during low demand in summer. The power system is now facing challenges to support low-carbon generation with intermittency. Design, operation and control of power system are now undergoing significant innovation of unprecedented scale seen since 1940s.

Since the last phase of developments in power systems has been driven by a strong interest in renewable generation, power electronics has become indispensable part to understand, model and analyze. Power electronics is the enabling link between variable frequency wind generations and DC generation from PV. The widespread availability of communication, smart measurements and computation has further thrown interdisciplinary theme in the power system, which was previously unknown. The advances in power electronics, from the use of thyristors to IGBTs, have brought about many innovations. The classical current source converters had many limitations, which were overcome by the newer voltage source converter type. The increased interest in larger meshed DC grids has led to the search of a DC breaker technology. As mentioned, communication has played a major role in the development of power systems. Smart metres have been rolled out in many countries, to gain an improved observability and controllability of the demand side. Control of demand has become a part of the solution for future power systems, especially with an increasing proportion of electrified transportation.