Contingency Evaluation of the Nigerian 330kv Transmission Grid
Chapter One
THE AIM AND OBJECTIVES OF THE STUDY
The aim of the thesis is to carry out contingency analysis at the various buses and transmission lines of Nigerian 330kV transmission grid.
The objectives of the study are as follow:
- Calculating the voltage magnitude in the network.
- Determining if the voltage variations are undesirable and not within the stipulated regulatory limit of ±5% of the declared voltage at the each of the buses of the transmission network.
- Determining the performance indices of transmission lines outage so that network weaknesses can be identified.
- Ranking the contingency according to its severity for priority attention of the System Engineer.
- Determining the effect of loss of one or more generators on the power system.
- Identifying the effect of loss of transmission lines on the performance of the power system.
CHAPTER TWO
LITERATURE REVIEW
Many interconnected power systems are increasingly experiencing abnormal high voltage or low voltage leading to system blackout [3]. Owing to this, several researchers have used different methods to proffer solution to the challenges facing the power system. However, the benefits of power system contingency evaluation for network expansion and power system stability have been treated in [4]. In their work, the authors used Newton Raphson Load flow method to carryout load flow analysis and determined the system line overload index (SLOI) of the various buses of the 330kV transmission grid of Nigerian power system thus, assessing the security level of the network. Further, in an attempt to give insight about contingency analysis, they enumerated the various methods of contingency analysis to include:
- AC load flow method.
- Z-Matrix method.
- Performance index method.
- DC load flow method.
However, of all the above listed methods, the AC load flow methods are considered to be deterministic and more accurate than DC Power flow methods which are simulated by modeling unlike the former which are carried out by the actual removal of the lines [4]. They however looked at the overloading at various transmission lines but could not bring to lime light the voltage instability of the network as well as the effect of power losses in the transmission lines.
Contingency selection was carried out by Roy and Jain [5] to determine active power performance index and reactive power performance index using Fast Decoupled Load Flow (FDLF) but left behind the voltage instability in the network. According to the increasing utilization in power system, the transmission line and power plants often operate in stability boundary and the system often loses its stability condition by overloading or disturbance. It was based on the above premises that the authors used maximum loading parameter point to analyze the static voltage stability using continuation power flow method.
The power flow study is seen as the most frequently carried out study performed by power utilities and it is required to operate at almost all the stages of power system planning, optimization, operation and control. In steady state security assessment of a power system, it is of paramount importance to determine the line flows and bus voltages at different loading conditions of a power system [6, 7]. In literature, several approaches such as PQ iteration method [8], distribution factor, the bounding method and the concentric relaxation method [9] have been proposed to estimate bus voltage in real time applications. The main objective of power flow (PF) studies is to determine the bus voltage magnitude with its angle in all the buses, real and reactive power flows (line flows) in different lines and the transmission losses occurring in a power system [10, 11].
A single line contingency was performed on Sudan National Grid using full AC load flow analysis and applying the simulation of lines and generators outage [1]. According to Eldeen et al [1] line power flow is said to be violated when the actual power flow post contingency exceeds the line flow limits which depends on the protection relay settings. Some contingencies lead to line flow violations, and some of them do not lead to any violations.
Sensitivity based wide performance indices with respect to outages have been presented in [12]. In the work, the Newton-Raphson power flow results were used to construct two kinds of performance indices, namely the real power performance index and voltage performance index, which reflect the degree of severity of contingencies. Moreover, full AC load flow was performed for each contingency case in order to study the contingency effect. The effectiveness of the proposed method was tested to demonstrate the contingency screening and thereafter ranked on a standard 6-bus and IEEE 14-bus systems. However, the time per iteration of this method is relatively higher when compared with FDLF method.
In the work of Sekhar and Mohanty [13], Newton-Raphson load flow method was used for the network analysis. Moreover, power system contingency ranking for the line outage, based on the active power and voltage performance indices was also exploited in the work.
However, when solving large-scale power transmission system, an alternative strategy for improving computational efficiency and reducing computer storage requirements is the FDLF method which makes use of an approximate version of the Newton-Raphson procedure with the inclusion of voltage stability constraint, the algorithm has helped to improve the voltage security of the system [14]. Also the transmission loss obtained with the proposed algorithm is less than that reported in the literature.
An expert system is developed to determine the steady state security of a power system after a contingency by which line and generator outages are considered [15]. For any load in the system, overloaded branches are determined by comparing the system load to three load levels closest to the actual system load and using generator shift or branch outage distribution factors. The expert system, in conjunction with the DC load flow, is designed to identify the harmless and harmful contingencies as well as the overload branches. If a contingency cannot be determined as harmless or harmful then an AC load flow was suggested. Turbo-Prolog is used for symbolic manipulations and reasoning. The developed system is applied to a 12 bus power system but the domain specific knowledge must be available for expert System to be applied.
The Tchebychev iteration of an indirect method was presented for solving a set of linear equations of fast decoupled power flow (FDPF) in the contingency screening [16]. Furthermore, the precondition technique was introduced to improve the convergence characteristics of the indirect method.
CHAPTER THREE
DATA COLLECTION AND METHODOLOGY
We exploited the use of the Fast Decoupled Load flow technique for the in-depth study of the load flow of the 330kV transmission line network based on its accuracy and time for convergence. However, the present-day trends are towards the development of iterative power flow programs where the planning engineer can modify data on a computer either in dialogue mode with the program or on graphic displays and then direct the program for solution of the problem. Based on the premise, certain tools like MATPOWER 4.1 and MATLAB 7.5 were used for the analysis of the 330kV transmission network. The MATPOWER 4.1 contains the script format for modeling the power network so as to obtain the real and the reactive power flow, power losses and voltage profile on the buses. The data collected for the purpose of the analysis was obtained from the report of the PHCN transmission station at Oshogbo, Nigeria.
METHODOLOGY
The collation of bus data and the transmission line parameters of 330kV transmission network was used in the Fast Decoupled Load Flow techniques to obtain buses whose voltage magnitude is lower or greater than a limit of ±5% of the declared value as well as the real power loss of the system. Besides, the active performance indices were calculated to discover the overloaded lines, lines with unacceptable voltage magnitudes and unacceptable losses. This involves identification of contingency that lead to the violation of the operational limits by single line contingency evaluation. The flow chart for fast decoupled load flow, contingency selection based on voltage violation, contingency evaluation using performance indices are respectively shown in figures 3.1, 3.2, 3.3 and 3.4 respectively. However, the generator data, load data, length of 330kV transmission grid, line data are tabulated in Tables 3.1, 3.2, 3.3 and 3.4 respectively.
CHAPTER FOUR
SIMULATION AND RESULTS
The simulation of the data was done with MATPOWER4.1. It is a script format that runs the power flow (Fast Decoupled Load Flow). The MATPOWER has the primary functionality of solving the power flow problems. It does this by:
- Preparing the input data
- Defining the relevant power system parameters
- Invoking the function to run the simulation
- Viewing and accessing the results that are printed to the screen and
- Saving the output data structures or files
PREPARING CASE INPUT DATA
The input data for the case to be simulated are specified in a set of data matrices packaged as the fields of a Matlab struct, referred to as a “Matpower case” and denoted by the variable mpc. The struct is typically defined in a case file called M-file whose return value is mpc struct.
CHAPTER FIVE
CONCLUSION
Considering the number of numerical iteration per time taken, Fast Decoupled load flow method has proven to be excellent in the load flow analysis of the 27-bus Nigerian 330kV transmission grid. The load flow studies identified buses with low voltages. At the outage of Kainji generator bus, it was observed that Birnin-Kebbi, Jos, Gombe, Kaduna, Katampe, Oshogbo, Ajaokuta, New Haven, Aiyede, Ikeja-West, Onitsha and Akangba have voltages less than 0.95pu which was regarded as low voltages. Power losses greater than 5% were also observed at Jebba TS to Oshogbo, Ikeja-West to Egbin, Ikeja-West to Benin, Oshogbo to Benin, Kaduna to Shiroro, Kano to Kaduna, Jos to Kaduna and Jos to Gombe. At the outage of Sapele GS and Calabar GS, low voltages were noticed in the same buses given above and also power losses greater than 5% in the same line above. However, a single line contingency evaluation shows that a transmission line from Kaduna to Shiroro has the highest severity index and hence ranked number one (1). The ranking continued until the line with the least performance index. Furthermore, it can be seen that the outages of one or more generators give low voltages at the same bus and power losses at the same transmission lines. Hence, it becomes expedient to compensate the lines.
RECOMMENDATION
We recommend an optimal placement of FACTS devices at the buses and transmission lines with excessive power losses, low voltages and high severity index in order to minimize real power losses, improve the voltage, strengthen the grid robustness and also reduce the frequent incidents of avoidable system collapse.
SUGGESTIONS FOR FUTURE WORK
- The complexities involved in the contingency analysis of a power system network leave much to be desired, so, a non-conventional approach should be exploited such as Artificial Intelligence that will carry out the analysis without much rigour.
- The work did not look at N-2 Secure otherwise known as double line contingency analysis.
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