Advance Voltage Stability Analysis For 132KV Transmission Network Using Voltage Stability Index

INTRODUCTION
1.1 BACKGROUND OF THE STUDY
Until early 20th century, the quality of power supply was not considered as important factor in power delivery. Utility companies only focused on achieving a power delivery state with little or no interruption. But with improvement in technology, paving way for development of very sensitive loads, and coupled with customer awareness, increased in electricity demand in homes, offices and industries, and inter-connection of electrical utility into complex grid etc, power system engineers were implored to consider power quality. Power quality as a term, defines a set of electrical boundaries within which a piece of equipment can function as intended without significant loss of performance or life expectancy (Sankaran t al., 2012). It entails delivering electric power with minimal distortions, and therefore, maintaining a near sinusoidal signal waveform at a frequency of 50Hz and at required load voltage.
Power Quality problems are manifested in voltage, current or frequency (Zahir et al, 2011). Examples include: voltage swell and sag, voltage fluctuation, harmonic distortions etc. Aside factors like power system faults, start up and shutdown of heavy equipment, switching operations etc, non-linear loads are identified as major cause of power quality problems. Power quality problems are global issues and exist in distribution systems of several countries of the world, including Nigeria (Zahir et al, 2011). The effects of power quality problems are enormous, ranging from equipment failure to equipment damage which can result in huge financial losses in process and automation industries. In Rumuola distribution system network, under-voltage and over- voltage are identified as major power quality problems (Irfan et al., 2013).
The need to mitigate power quality problems and maintain power of good quality has brought power system engineers, equipment manufacturers, researchers and statutory bodies to a focal point of methodology development. Today, several methods exist to improve the quality of power to sustain the ever increasing applications of sensitive and non-linear loads in distribution network. Conventionally, Synchronous condenser, capacitor banks, static VAR compensators (SVCs), self-commutated VAR compensators etc. are used to control reactive power and improve power factor, though with drawbacks such as instability problems, generation of high transient during connection and disconnection etc. [9]. More recently, Custom power devices such as distribution static compensator (DSTATCOM), unified power quality conditioner (UPQC), dynamic voltage restorer (DVR) etc were found to be improved methods for power quality control. Ogunyemi and fakolujo considered the relevance of custom devices in tackling power efficiency problems in Nigeria distribution network stating that such devices have been widely used in distribution network of developed countries. However, the performance of custom power devices is dependent on the type of controller employed. Proportional integral (PI), proportional integral differentiators etc are effective but slow in response and perform poorly under parameter variations. Artificial intelligent (AI) controllers such as Artificial Neural Network (ANN), fuzzy logic etc are proposed by researchers as they offer better performance in terms of response time and operation under dynamic loads (Ogunyemi et al., 2012).
Several authors have researched on mitigating power quality problems in distribution system network using DVR. Harmonics and under-voltage compensation using DVR was studied by Sundarabalan et al (2014) using ANN controller based on park’s transformation strategy. The ANN controller was trained off-line with data from a proportional integral controller. In another study, Shairul, et al simulated the performance of DVR using PSCAD (Shairul et al., 2014). Raunak, et al (2014) in their study of DVR performance on sag and swell mitigation applied PI controller and unit vector extraction control scheme. In these studies, a simple distribution network composed of two feeders fed from a substation was employed, and voltage sag was simulated by different conditions. The results obtained showed the effectiveness of DVR in under-voltage and over-voltage mitigation. This paper seeks to improve power quality by mitigating under-voltage and over-voltage using ANN based controller model of dynamic voltage restorer (DVR).

1.2 PROBLEM STATEMENT
Current civilization is susceptible to case of power system blackout, the consequences of systems failure are social and economic as well. Even short disturbance can be harmful for industrial companies, because restarting of process might take several hours. In recent years, voltage instability has been responsible for several major network collapses. There are many method that has been used in the past for the improvement.
Conventional voltage stability improvement methods such as capacitor banks, reactors and transformers can be used to provide steady state voltage control. However, these devices are based on electro-mechanical control among other drawbacks explained later thus impeding high speed voltage control.
Using artificial neural network provide a better adaptation to varying operational conditions and improvement on the usage of existing installations in power systems by using power electronic controllers. Their main advantages over the conventional methods are that the devices are both dynamic-fast controllability using power electronics- and static-no moving parts to perform the dynamic controllability.
This research sought to come up with a new way of voltage stability improvement using artificial neural network by solving the challenge of reactive power absorption and generation for real-time control.

1.3 AIM AND OBJECTIVES OF THE STUDY
Voltage stability analysis is important in power system in order to maintain the power system equilibrium. The main aim of this work is to carry out a study on the advance voltage stability analysis for 132kv transmission network of the Nigerian eastern zone using voltage stability index. The objectives of this research are:
i. To analyze the advance voltage stability improvement of a power system using voltage stability index method.
ii. To perform a security constrained load flow solution on the system
iii. To perform a cost-benefit analysis of the developed voltage control system.

1.4 SCOPE OF THE PROJECT
Power systems are complex systems consisting of large number of generating units and interconnected network of transmission lines. The voltage stability is an issue of prime importance in this complex power system network since the demand for electric power is increasing drastically. The control of reactive power in the transmission lines will enhance the voltage stability of the power system network.

1.5 SIGNIFICANCE OF THE STUDY
This research work will throw more light on how voltage stability index can be used to improve power stability in a 132kv power system.
This research sought to teach the reader on how to come up with a new way of voltage stability improvement thereby solving the challenge of reactive power absorption and generation.

1.6 PROJECT JUSTIFICATION
Power systems are increasingly becoming more overloaded and constantly being operated close to their voltage stability limits. Voltage stability is the ability of a power system to maintain acceptable voltage levels under normal operating conditions and after being subjected to disturbances such as a sudden increase in load or loss of a major generation plant. Major national power outages have been documented in the recent past in countries in Africa such as in Nigeria.
The rise in the use of voltage stability index methods for voltage stability improvement has been a growing trend due to the huge capital outlay required to construct new transmission and distribution lines, pressure on land as well as environmental concerns worldwide.
A voltage stability index method will go a long way in voltage stability improvement. This research uses voltage stability index which mimic biological nervous systems as a way of trying to replace human operators who are at times slow to relay and act on information on system voltage profiles thus leading cascaded system voltage collapse.

1.7 THESIS ORGANIZATION
Chapter 1
This chapter presents an introduction of the research work, outlines the problem statement, gives a justification for the research work and finally the goals of the work.
Chapter 2
This chapter presents a literature review on voltage stability analysis.
Chapter 3
This chapter gives the methodology followed in carrying out this research work.
Chapter 4
This chapter presents the results obtained and an analysis and discussion of the same against the objectives of the research.
Chapter 5
This chapter presents a conclusion of the work and gives recommendations and/or gaps for future research.