CHAPTER 4: EFFECT OF RENEWABLE GENERATION TECHNOLOGIES ON POWER
4.5. Overview of Power System grid
The electrical power grid is an interconnected network that delivers the generated power to the consumers. The power grid consists of generating stations that produce electrical power, high voltage transmission-lines that carry power from distant sources to distribution centers, and distribution lines that connect individual customers to the grid. The general structure of the electric power system as treated in previous research on power systems is as shown in Figure 4.1. The electric power is generated in large power stations at relatively small different locations. This power is then transmitted and distributed to the end-users, typically simply referred to as load/consumers (Bollen & Gu, 2006).
In South Africa, the power grid consists of the generating station (generation plant), transmission system, distribution system, and reticulation network. Power generating stations are situated at a feasible location closer to the availability of the source. In all industrialized countries, this remains the structure of the power system (Bollen & Gu, 2006). Hence, the generation plant is often located relatively away from populated areas. This is practical because the transmission of electrical power over longer distances is considered more economical (Bollen & Gu, 2006).
The electrical source includes a hydroelectric power plant, off-shore wind farm, and more centralized generation such as coal and nuclear plants. At transmission system is therefore used to transmit generated electrical power over long distance to populated cities. A world- wide transmission system connects the large generator stations. The transmission system allows the sharing of the resources from the various generator stations over large areas.
Distribution system and reticulation are used to distribute the power to consumers at appropriate voltages. Similarly, distribution networks transport the electrical energy from the transmission sub-stations to the various loads. There is no complete criteria to distinguish between distribution and transmission networks. Some countries use the term sub-
78 transmission networks or an equivalent term to refer to the networks around big cities that have a distribution system structures (Bollen & Gu, 2006).
Figure 4.1: Traditional power grid, characterized by generation, transmission, distribution and reticulation of electricity to end-use consumers. Image Source (EPRI, 2014)
Due to several developments during the last decade, the model in Figure 4.1 no longer fully holds. In most countries, generation is completely integrated and deregulate as in the case of USA and Europe. This is, intended for effective management of the energy economy.
Transmission and distribution are often split into separate companies. Each company is economically independent, even where it is electrically an integral part of a much larger system.
The need for environmentally friendly energy has led to the introduction of smaller generator units. Clean renewable generation technologies or embedded generation or distributed generations are often no longer connected to the transmission system but to the distribution system. A more modern way of looking at the power system resulting from these developments is shown in Figure 4.2 below. The electric power network no longer only transports energy from generators to end-users but instead enables the exchange of energy between Independent Power Producer (IPP) and the national electrical grid.
The downstream of network in Figure 4.2 could be a transmission network or a distribution network or an industrial network or any other network owned by a single company. In South Africa, IPP’s are currently part of the distribution system generating electricity for distribution at different generation sites and are integrated to the electrical grid to increase generation
79 capacity. However, in countries like Rwanda and other African countries, IPP’s are generating electricity for transmission and distribution to meet the national energy capacity demand programs. This disparity in T&D exhibits the clear needs for robust and efficient networks.
The aim of the T&D network is to only transport the electrical energy, or in economic terms, to enable transactions between generators and consumers. The diverse role of renewable generation integration plays a major role in T&D systems of the electricity energy market economy. However, the major concern which places enormous constraints on the development of REG in Africa is the limited ability of the network to transport electricity.
According to Eberhard, (2015), the lack of generation capacity is due to aging networks and virtually none existent networks in many African countries and is a relating deficiency to Africa’s energy market economy.
Figure 4.2: Concept of integrated grid with multiple renewable generation sites for distributed energy resources, networked with other points of generation as a distributed energy network.
80 Moreover, research by Passey et al., (2011), suggests that effective and significant utilization of intermittent renewable generation located away from major load centers cannot be accomplished without significant additions to the transmission system.
As renewable generation integration continues to develop, the clear solution to integration impact will be found in collaboration between the network service providers (NSP) and IPP operator considering various technical and economic constraints (Eberhard, 2015). Another possibility for an effective electric T&D system is the modernization of the network which is impacted by the economic cost.