30-11-2012, 12:36 PM
FUZZY APPROACH TO FAULT CLASSIFICATION FOR TRANSMISSION LINE PROTECTION
FUZZY APPROACH.doc (Size: 1.73 MB / Downloads: 53)
Abstract
This paper presents a scheme to real time fault location And classification of transmission lines using fuzzy logic technique. Protection algorithm is based on the traveling waves present on major transmission lines after the incidence of fault.
The time interval between the initial traveling wave propagated from a fault and a later wave resulting from a reflection, at the fault position is used to find the location of fault..A possible application based on modal analysis to detect the type of fault is also proposed.
This paper shows that a fuzzy approach can be useful in transmission line protection, whenever fuzzy decisions have to be under taken. Simulation results are used to illustrate the basic features of performance of the new scheme on a 22OkV-transmission line.
INTRODUCTION
NEED FOR PROTECTIVE SYSTEMS:
An electrical power system consists of generators, transformers, transmission and distribution lines etc. Short circuits and other abnormal conditions often occur on a power system. The heavy current associated with short circuits is likely to cause damage to equipment if suitable protective system is not used. The short circuits are called faults by power engineers. A heavy short circuit may cause a fire. The system voltage may reduce to a low level and the generators may lose synchronism. Thus a heavy short circuit may cause a total failure of the system.
High impedance faults albeit uncommon, must nonetheless be accurately detected and removed. This is more so in view of the fact that apart from threatening the reliability of electric power supply, these faults pose a risk of fires and endanger life through the possibility of electric shock.
A protective scheme includes circuit breakers and protective relays to isolate the faulty section of the system from the healthy sections. A circuit breaker disconnects the faulty element when it is called upon to do so by the protective relay. A protective relay senses the abnormal conditions on a power system and detects a fault.
NATURE AND CAUSES OF FAULTS
Faults are caused either by insulation failures and conducting path failures. Most of the faults on transmission and distribution lines are caused by over voltages due to lightning and switching surges, or by external conducting objects falling on overhead lines. Sometimes, certain foreign particles, such as fine cement dust or soot in industrial areas or salt in coastal areas or any dirt in general accumulates on the surface of string and pin insulators. This reduces their industrial strength and causes flashovers. Tree branches or other conducting objects falling on the overhead lines also cause short circuits. Birds may also cause faults on overhead lines if their bodies touch one of the phases and the earth wire. Other causes of faults on the overhead lines are: direct lightning strokes, aircraft, snakes, ice and snow loading, abnormal loading, storms, earthquakes, creepers, etc. In the case of cables, transformers, generators, and other equipment, the causes of faults are: failure of the solid insulation due to ageing, heat, moisture or over voltage, mechanical damage, accidental contact with earth or earthed screens, flashover due to over voltages etc.
EFFECTS OF FAULTS
A fault if nucleated has the following effects on a power system.
1. Heavy short circuit current may cause damage to equipment or any other element of the power system due to overheating and high mechanical forces set up due to heavy current.
2. Arcs associated with short circuits may cause fire hazards.
3. There may be reduction in the supply voltage of the healthy feeders, resulting in the loss of industrial loads.
FAULT STATISTICS
For the design and application of a protective scheme, it is very useful to have an idea of the frequency of occurrence of faults on various elements of a power system. Usually the power stations are situated far away from the load centres, resulting in hundreds of kilometres length of overhead lines being exposed to atmospheric conditions. The chances of faults occurring due to storms, falling of external objects on the lines, flashovers resulting from dirt deposits on insulators, etc., are greater for overhead lines than for other parts of the power system. Table 1.1 gives an approximate idea of the fault statistics.
FUNCTIONS OF A PROTECTIVE SYSTEM:
A protective system protects the power system from deleterious effects of a sustained fault, which occurs as a random event. If some faulted power system component (line, bus, transformer, etc.) is not isolated from the system quickly, it may lead to power system instability or break –up of the system through the action of other automatic protective devices.
A protection system –consisting of one or several relays –is made responsible for all faults occurring with in the zone of protection. When such a fault occurs, the protection system will activate trip coils of circuit breakers there by isolating the faulty portion of the power system inside the boundary. Usually – the zones of protection are defined by circuit breakers. If the zone of protection does not have a circuit breaker at its boundary, the protection system must trip some remote breakers to deenergize the faulted zone.
PROTECTIVE RELAYS:
The capital investment involved in a power system for the generation, transmission, distribution of electrical power is so great that proper precautions must be taken to ensure that the equipment not only operates as nearly possible to peak efficiencies, but also that it is protected from accidents. The purpose of the protective relays and protective relaying systems is to operate the correct circuit breakers so as to disconnect only the faulty equipment from the system as quickly as possible, thus minimizing the trouble and damage caused by faults when they occur.
EVOLUTION OF PROTECTIVE RELAYS:
The earliest relays to be used in transmission line protection were all electromechanical.The very first relays were based on the over-current principle which was introduced around1902. The inverse time-current relationship was suitable for time graded over-current discrimination systems. These early devices not only had to detect fault conditions, but also had to generate sufficient torque to trip the breaker on which the system was fixed. The latter requirement placed very severe restrictions on the sensitivity of these devices
These electromechanical distance relays later achieved very high precision in the form of induction cup mho relays. The mho relay gave a closed characteristic of the fault impedance locus and therefore allowed discrimination against faults in other phases.Mho relays were therefore mainly incorporated as starting units in the majority of distance relaying schemes. The mho characteristic is still widely applied in distance relays.
Additional Fuzzy Operators
In this case, you defined only one particular correspondence between two-valued and multivalued logical operations for AND, OR, and NOT. This correspondence is by no means unique. In more general terms, you are defining what are known as the fuzzy intersection or conjunction (AND), fuzzy union or disjunction (OR), and fuzzy complement (NOT). The classical operators for these functions are: AND = min, OR = max, and NOT = additive complementA single fuzzy if-then rule assumes the form if x is A then y is B where A and B are linguistic values defined by fuzzy sets on the ranges (universes of discourse) X and Y, respectively. The if-part of the rule “x is A” is called the antecedent or premise, while the
then-part of the rule “y is B” is called the consequent or conclusion. In MATLAB® terms, this usage is the distinction between a relational test using “==” and a variable assignment using the “=” symbol. A less confusing way of writing the rule would be.If service == good then tip = average. In general, the input to an if-then rule is the current value for the input variable (in this case, service) and the output is an entire fuzzy set (in this case, average). This set will later be defuzzified, assigning one value to the output. The concept of defuzzification is described in the next section. Interpreting an if-then rule involves distinct parts: first evaluating the antecedent (which involves fuzzifying the input and applying any necessary fuzzy operators) and second applying that result to the consequent (known as implication).