CHAPTER ONE
INTRODUCTION
- Background of Research
Security assessment is defined as the real time analysis procedures by which the security of a power system is measured or assessed [1]. On the other hand, power system security is the ability of the system to withstand major disturbances [1],[2]. Power systems originally existed as small, individual, self-sufficient units where the power production and consumption balanced out. In the event of a severe failure, a system collapse was unavoidable and meant a total blackout and interruption of supply to customers. But the restoration of the whole system and synchronization of its generators were relatively easy due to its small size.
Recently, owing to the deregulation of the power industry, technological advances and the consequent competitive power market, network complexity and size have changed tremendously. The deregulated electric power utilities are being restructured and operated as distinct generation, transmission and distribution companies and the responsibility of maintaining the security of the overall system is shared by all involved companies, instead of by a single utility.
The need to meet up with the main objectives of an electric power utility (which is to ensure balance of generation and load and supply of electric power/energy as economically as possible, while maintaining acceptable levels of service quality) and the rising importance of electricity make the security of supply and overall power system security a serious issue [3].
The Nigerian government adoption of the policy of privatization of its electric power sector has attracted private investors who embark on the construction of independent power plants (IPPs) that are connected to the grid [4]. This falls under the subject of distributed generation (DG). Examples include an already existing power plant at Okpai in Delta State built by Agip and another under construction at Aba in Abia State by Geometric.
Incorporating DG into the power system poses numerous challenges in terms of connection, protection coordination and power quality [5]. However, DG is at its developmental stage in Nigeria and thus there is need to evaluate its contribution to network security. It is therefore in an attempt to assess the impact of DG on the ability of power system to withstand disturbances that this research has been embarked on as a good beginning point in planning a well coordinated and secure network and service.
Explicitly, this research effort is geared towards the evaluation of the contribution of DG to the steady state security of network components (buses and lines) using the voltage and current-based loading indices respectively and overall network steady state security following single independent line outages. This will be done by employing a study methodology that is analytical and comparative.
In addition, recommendations will be made for further research in this field.
1.2 Statement of the Problem
Electricity generation in Nigeria began in 1896. NESCO commenced operation as an electric utility company in Nigeria in 1929 with the construction of a hydroelectric power station at Kurra falls near Jos. ECN was established in 1951, while the first 132kV line was constructed in 1962, linking Ijora and Ibadan Power Stations. NDA was established in 1962 with a mandate to develop the hydropower potentials of the country. However, the merger between ECN and NDA in 1972 gave birth to NEPA. The National Electricity grid consisted of eight functional generating stations (three hydro and six thermal) with total installed generating capacity of 5906MW [6].
In 1998, NEPA ceased to have an exclusive monopoly over electricity generation, transmission, distribution and sales. This is in line with the Nigerian Government adoption of a policy of privatization of its electric Power Sector and that transformed NEPA to PHCN [4]. Recently, steps have been taken in the direction of power sector reforms.
Despite these efforts, there is no doubt that power supply in Nigeria remains erratic, marked by voltage fluctuations (brownout), indiscriminate and unpredictable duration of power outages (blackouts). The measure of network reliability shows the amount, duration and frequency of outage which occur due to deficiencies in network components which cause the inability of the network to withstand major disturbances. It is quite disappointing that the reliability of power generating stations in Nigeria though estimated to be quite low, cannot easily be evaluated. In fact, according to [7], there appears to be no standards or benchmarks, but rather unreliable and inconsistent estimates of probable job duration, performance and maintenance reports. Hence, it is obvious that the security of the Nigerian Power system and its supply is in question.
Again, the electric power sector all around the world is undergoing a series of changes which impact on the system performance and the Nigerian Power sector is not left out. Among these changes, the most well known is the industry restructuring towards a market-based environment that has pronounced the interconnection of DG.
Though DG helps in reducing the stretch on weak electricity grids [6],[8], it is not all-round beneficial to the network as elucidated in section 2.11. The central role of electricity implies that its network security remains a most important aspect of its operation and cannot be compromised in a market-driven environment.
Therefore, there is need for the assessment of the impact of DG on the network security especially at the distribution level for some major reasons. First, DGs are primarily connected at this level. Secondly, the low voltage ends of distribution systems are usually not connected to the supervisory control and data acquisition (SCADA) systems. As such, data gathering required for the control of distribution systems is therefore difficult [6]. Thus, adequate preventive and control measures are seldom implemented on Distribution networks. Thirdly, the networks have high impedance lines and are usually radial, allowing one-directional flow of power. Lastly, distribution systems are usually not designed for the connection of power generation devices and suffer most from illegal load tap-off.
1.3 Purpose of Study
Electricity is an important component of energy mix in Nigeria and the most convenient form of energy. Thus, an evaluation of the security of its network (especially at the distribution level) and the determination of the impact of single independent line outages (based on the (n-1) criterion) is essential for planning and adequate supply of power to consumers.
Hence, the purpose of this study is to carry out steady state security analysis of a distribution network with distributed generation using analytical method.
1.4 Research Scope and structure
In this work, a twenty-bus system of Emene 11kV distribution network in Enugu is selected as a case study. The result of the study is extended to give insight into the contribution of DG to the steady state security of larger radial distribution systems.
This dissertation is structured into six chapters. Chapter 1 covers introduction and background information. Chapter 2 reviews the concepts of power system security and Distributed Generation. Chapter 3 embodies Mathematical modeling for steady state security assessment. A practical case with software simulation is done in Chapter 4. Chapter 5 discusses the results obtained in Chapter 4. This work concludes in Chapter 6 and recommendations for further research are enumerated.
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