ABSTRACT
In this thesis, a novel approach for the protection of transmission lines which utilizes only coefficient energy for both detection and classification is proposed. The fault current signals generated by workspace on MATLAB simulation model have been analyzed using Daubechie-4 (d4) mother wavelet at 7th level decomposition with the help of Wavelet Toolbox embedded in MATLAB. A case study of 132kV, 160km transmission line has been used to test the novel approach. The value of the coefficient energy of the current signals gives the indication of fault and no- fault conditions. The energy of the three phase current signal (A,B,C) at 7th level decomposition were calculated as 0.1559×10-5, 0.1328 x10-5, 0.1737 x10-5 (for normal condition), 6.4200 x10-5, 1.7730 x10-5,
1.6660 x10-5 (for A-G fault), 667.1000 x10-5, 700.9000 x10-5, 0.7860 x10-5 (for AB-G fault), 677.8000
x10-5, 689.9000 x10-5, 0.1740 x10-5(for A-B fault), 885.6000 x10-5, 898.3000 x10-5, 832.7000 x10-5(for ABC fault). Also, the coefficient energy ratios were calculated to help classify the faults. The total ratio of the coefficient energies of the three phases were found to be approximately 3.4819 (for normal condition), 5.9177 (for A-G fault), 1741.4580 (AB-G fault), 7861.3448 (for A-B fault), 3.1423 (for ABC fault). Like the coefficient energy, the ratio was found to be increasing as the severity of the fault increases, except for L-L-L fault. Hence, both coefficient energy and ratio were employed in fault classification. With the approach presented in this work, ten classes of fault (A-G, B-G, C-G, A-B, B-C, A-C, AB-G, BC-G, AC-G & ABC) could be correctly identified and classified within fault duration of 0.085 seconds. The results therefore, demonstrate the proposed approach to be fast and reliable.
CHAPTER ONE: INTRODUCTION
1.1 Background
Electricity is a basic necessity of life in that social and economic development of any country is highly dependent on the availability of power supply. Without adequate supply of power, businesses will not be full, companies will not produce, unemployment will set in and ultimately the standard of living will dwindle. If everyone has to own generating unit, environment will not be conducive. But not even all homes can afford private generating unit. The most economical is to pull power from the national power system.
Power system is a very robust system, probably the largest and most complex industry in the world. It is comprised of generation, transmission and distribution. These parts of the power systems are interconnected and the failure of any might affect the performance of the other. An important objective of all the power systems is to maintain a very high level of continuity of service, and when abnormal conditions occur, to minimize the outage times. It is practically impossible to avoid consequences of natural events, physical accidents, equipment failure or
disoperation which results in the loss of power and voltage dips on the power system.
Power generation
System
Power Transmission
System
Power Distribution
System
Fig 1.1: Power System Basic Blocks
Electrical Supply is a big and discrete investment that shows significant economies of size and takes time to build. Therefore, electrical outages are expensive. Short term outages can destroy production and leisure while long term can be a serious impediment to economic growth
and development. Several approaches are employed to guard the power systems against failure and to protect the components of power system in the event of faults. Power system protection is a branch of Electrical power Engineering that deals with the protection of Electrical power systems from faults through the isolation of faulted parts from the rest of the electrical network [1]. Power system protection becomes easy if the possible causes of faults are known and proper protection scheme employed.
Overview of Transmission Company of Nigeria’s (TCN) network
Power generations in Nigeria are not evenly located. The thermal generation is located in the south of the country, generally near to the sources of gas, while the hydro generation is located further north at Jebba, Kainji and Shiroro. Overall, there is a shortage of generation in the North. Therefore, transmitting power from south to the North requires a very high voltage over a very long distance. The transmission system in Nigeria comprises 330 kV and 132 kV circuits and substations with System nominal frequency of 50 Hz. The distribution networks comprise 33 kV,
11 kV and low voltage circuits. The existing TCN 330 kV transmission system uses double and single circuit twin Bison Aluminium Conductors Steel Reinforced (ACSR) overhead lines. Bison is a 350 mm2 conductor, with a continuous current rating of about 680 A per conductor, which equates to a continuous maximum thermal limit for each circuit of 777 MVA
Nigeria Transmission Network is connected with other neighbouring countries via the following interconnectors:
Ikeja West(Nigeria) – Sakete (Benin Republic) 330KV Single Circuit Transmission line which is part of WAPP South Core
Benin Kebbi (Nigeria) – Niamey 132kV Single Circuit Transmission line expected to be upgraded to 330KV Transmission line which is part of WAPP North Core
Katsina (Nigeria) – Gazaoua (Niger) 132kV Single Circuit Transmission Line
These connections are not synchronous but just Nigeria feeding isolated loads. Figure 1.2 shows the Nigeria grid system.
Fig. 1.2: National Grid System. Source: [2]
1.2 Statement of research problem
Reports from world bank in Vanguard Newspaper of 13th September, 2015 shows that Nigeria has lost a whopping sum of about $100b in just one year to daily black out. The Nigeria power transmission network is characterized by prolonged and frequent outages. Also planned outage on the 132kV takes only 7% while the remaining 93% were due to either forced outages
or emergency/urgent outages [2]. Short-circuit currents in power systems are harmful for two reasons- the first is that even a short time flow of heavy current will overheat the equipment, the second is that the flow of short-circuit currents through the current carrying parts produces forces of electrodynamic interaction which may destroy or damage the equipment. Mostly 80-90% of the fault occurs on transmission line and rest on substation equipment and bus-bars combined [3]. These situations are not only as a result of inadequate power generation but also as a result of inefficient and improper coordination of protection schemes in the power network.
The key challenge to the protection of transmission lines lies in reliably detecting and isolating faults, compromising the security of the power system [3]. The time required in determining the fault point along the transmission line affects the quality of power to be delivered. Hence, a well coordinated protection system must be provided to detect & isolate various types of faults rapidly so that damage and disruption caused to power system is minimized.
1.3 Aims and objectives of the research
The aim of the research is to provide a new approach for protection of 132kV lines. The specific Objectives of this research include:
To detect fault cases on transmission line within shortest possible time by using Wavelet
Transformation technique to analyze the fault signal.
To classify the fault based on number of phases that constitute the fault. For instance, single line-to- ground fault, line-to-line fault, double line-to-ground fault etc.
To improve on the reliability of the power system by prompt handling of faults on the transmission network
1.4 Scope of the research
This research is centered on the development of an approach for efficient handling of faults on the transmission line. Therefore, prevention of fault occurrence on the transmission line such as shielding the line from lightening strokes is outside the scope of this work. However, due to the effectiveness of the proposed approach, fault clearing time will be drastically reduced. This work here focused only on faults occurring on the transmission lines. Faults occurring on generators, transformers, bus bars e.t.c are outside the scope of this work. However, the technique developed can still be employed to detect other faults in other parts of the power systems. This research is carried out based on 132kV network but the approach can still be promising for voltages higher than 132kV in any power system but may not be satisfactory for voltages below 132kV.
The research work here does not replace the conventional protection elements such as relays and circuit breakers but increases the speed with which they receive functional commands.
1.5 Significance of the research
This research will be of immense relevance to any power utility especially in a system where system automation has not grown much. Transmission Company of Nigeria (TCN) will benefit much from this research work if the idea in this work is implemented in her network. The approach developed in this work will be of enormous use for engineers in the control centers and in a SCADA system. It will also be useful to engineers in academic fields and research institutes. This work is not only useful in engineering fields but also in any discipline where denoising (removal of noise signals) is of interest. As this approach is still new, potential findings abound for anyone researching in the area.
1.6 Organisation of Thesis
This work is organized in five chapters. Chapter one presents a general introduction of the work. Chapter two presents the various kinds of faults, the Protection techniques that are currently available and general review of relevant literature materials on the subject area. Chapter three deals with the modeling of 132kV transmission line case study. Chapter four deals with MATLAB/SIMULINK simulation of the different fault types on the network. Wavelet decomposition of the resulting signals, fault detection and fault classification are also presented in this chapter. Conclusion is drawn and necessary recommendation made in chapter five.
This material content is developed to serve as a GUIDE for students to conduct academic research
A NOVEL APPROACH FOR POWER SYSTEM PROTECTION IN HIGH VOLTAGE POWER SYSTEM AT 132KV
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