1.3 Dissertation scope and objectives . . . 5 1.4 Dissertation organisation . . . 6
1.1 Background
The sustainability of modern society is heavily dependent on a secure and accessible supply of energy [63]. Power utilities have become one of the most crucial resources in any nation’s economy and therefore the operations planning of these utilities is of utmost importance [104]. Such operations planning is particularly challenging in the context of developing countries, because the electricity demands in these countries typically increase rapidly due to fast evolving economies [12]. Not only are electricity utilities pressured to meet the ever-changing demands in such countries, they are also pressured to remain current in respect of global energy policies that are currently pushing for “greener energy.” This places additional strain on the capital reserves of developing countries, because cleaner energy sources typically still come at a much greater cost than the more traditional methods of electricity generation [63].
In South Africa, which is classified as a developing country [119], Eskom is the sole power utility and was established in 1923 as the Electricity Supply Commission. It was then converted into a public, limited-liability company in 2002 that is wholly owned by the government of South Africa [75]. Today Eskom is recognised as one of the top twenty power utilities in the world, based on generation capacity, with a net maximum self-generated capacity of approximately 44 184 MW. Eskom supplies more than 45% of the electricity consumed in Africa and supplies approximately 96% of South Africa’s electricity [76]. The production and consumption of power in South Africa over the past 17 years are presented in Figures 1.1(a) and 1.1(b), respectively. Although almost 86% of the total installed generation capacity in South Africa is coal-fire based, alternative forms of renewable energy sources are constantly researched by the utility. The combination of the various technologies used to generate electricity is called the plant mix which, for Eskom, is shown in Table 1.1 [76]. As may be seen in the table, Eskom utilises six technologies in its electricity generation operations. Power stations are either classified as base load stations or as peak demand stations. Base load stations are stations that operate every day
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 180 200 220 240 260 280 Year Pro duction (TWh)
(a) The power production in South Africa over the past 17 years [72]
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 140 160 180 200 220 Year Consumption (TWh)
(b) The power consumption in South Africa over the past 17 years [72]
Figure 1.1: The energy production and consumption of South Africa over the past 17 years.
of the year, whereas peak demand stations only operate when the demand exceeds the supply capability of the base load stations.
Coal-fired power stations use coal as their primary fuel source. These stations are the work- horses of the South African power utility and are required to operate 24 hours a day in order to meet the country’s energy demand, with the exception of being offline when scheduled for planned maintenance or when failures occur.
Table 1.1: The plant mix of Eskom’s electricity generating technologies as on 4 October 2017 [76].
Type # Power Plants Type Plant mix
Coal-fired stations 13 Base Load 85.43%
Nuclear stations 1 Base Load 4.32%
Hydro stations 2 Peak Demand 1.36%
Pumped storage schemes 2 Peak Demand 3.17%
Gas fired stations 4 Peak Demand 5.49%
Africa’s first and only nuclear power station, Koeberg, has a total installed capacity of 1 910 MW and is located in Melkbosstrand within the Western Cape [76].
The third generation technology that Eskom employs is hydro energy power stations. These peak demand stations capture the energy of moving water from dams or in rivers and convert the kinetic energy to electrical energy. The two hydro power stations of Eskom are located in the Gariep dam near Norvalspont and in the Vanderkloof dam near Petrusville, respectively [76]. Another technology which also uses the kinetic energy of moving water is a pump storage scheme. This type of facility is also classified as a peak demand station. These stations work on the same principle as hydro power stations, but reuse the water that was used to generate electricity by pumping it back to a storage reservoir up-river during off-peak times to be reused during subsequent peak demand times [77].
The plant mix also contains four gas turbine power stations which have very quick start-up times, but also very high operating costs due to their use of kerosene and diesel as primary fuel sources. These stations are therefore only used during peak demand periods and during emergencies.
Eskom has additionally invested in two wind farms, one of which has already been in operation since 2002 and another which came into full operation early in 2015. The first wind farm consists of only three wind turbines with an installed capacity of merely 3 MW. The second wind farm, which is situated near Vredendal in the Western Cape, consists of 46 wind turbines with a combined installed capacity of 100 MW [76].
During 2015, the electricity system of South Africa was severely challenged and experienced a tightly constrained demand/supply balance, which put the power system at risk in respect of both its adequacy and reliability. This situation may be attributed to the fact that during the 1980s there was an excess supply of electricity due to an electricity generation expansion programme launched by Eskom during the late 1970s. As a result, little or no investments were made in the generation expansion of Eskom during the 1990s and early 2000s [125].
Also contributing to the highly constrained South African energy system, was the higher than expected demand since 2008. This caused nationwide blackouts and since then strain on the system has only recently decreased. Following this, Eskom introduced “load shedding” to the nation. Load shedding involves a series of planned rolling blackouts that follows a rotating schedule. It is used to decrease the demand during periods when Eskom cannot meet the required demand and the short supply threatens the integrity of the nation’s electricity grid [103].
Another reason for the situation that Eskom found itself in during 2015, is inadequate mainte- nance planning. It was recognised at the quarterly State of the System briefing held on January 15th, 2015 that a major reason for the previous South African energy situation is that Eskom had not performed the necessary maintenance on the power generating units (PGUs) of its power plants [89]. The required maintenance downtimes of PGUs had been postponed on a continual basis due to the high energy demand experienced. Although Eskom had a specific maintenance philosophy in place, it had not remained completely faithful to this philosophy and was therefore faced with massive challenges [89].
A power utility’s ability to satisfy energy demand can be influenced significantly by unexpected breakdowns of PGUs. In most cases, such unexpected failures are also much more expensive to repair than taking planned preventative maintenance action. Maintenance of ageing PGUs is, however, often neglected due to high energy demand and low system capacity, as seen in the case of Eskom. The typical objectives pursued in the design of PGU maintenance schedules do not take these difficulties into account. Two new scheduling criteria are therefore proposed in
this dissertation in order to explore to what extent generator maintenance scheduling (GMS) optimisation can contribute to the reliability of a power utility’s operations.