05-05-2012, 04:29 PM
AUTOMATIC FAULT DETECTION AND ISOLATION IN TRANSMISSION LINES
AUTOMATIC FAULT DETECTION AND ISOLATION IN TRANSMISSION LINES.doc (Size: 157.5 KB / Downloads: 124)
INTRODUCTION
Any abnormal conditions which causes flow of huge current in the conductors or cable through inappropriate paths in the circuit can be defined as a fault. In normal operating conditions all the circuit elements of an electrical system carry currents whose magnitude depends upon the value of the generator voltage and the effective impedances of all the power transmission and distribution system elements including the impedances of the loads usually relatively larger than other impedances.
Modern electric systems may be of great complexity and spread over large geographical area. An electric power system consists of generators, transformers, transmission lines and consumer equipment. The system must be protected against flow of heavy short-circuit currents, which can cause permanent damage to major equipments, by disconnecting the faulty section of system by means of circuit breaker and protective relaying. Such conditions are caused in the system accidentally through insulation failure of equipment or flashover of lines initiated by a lightning stroke or through accidental faulty operation.
The safe disconnection can only be guaranteed if the current does not exceed the capability of the circuit breaker. Therefore, the short circuit currents in the network must be computed and compared with the ratings of the circuit breakers at regular intervals as part of the normal operation planning.
The short circuit currents in an AC system are determined mainly by the reactance of the alternators, transformers and lines upto the point of the fault in the case of phase to phase faults. When the fault is between phase and earth, the resistance of the earth path play an important role in limiting the currents.
Balanced three phase faults may be analyzed using an equivalent single phase circuit. With asymmetrical three phase faults, the use of symmetrical components help to reduce the complexity of the calculations as transmission lines and components are by and large symmetrical, although the fault may be asymmetrical. Fault analysis is usually carried out in per-unit quantities as they give solutions which are somewhat consistent over different voltage and power ratings, and operate on values of the order of unity.
In case of circuit breakers, their rupturing capacities are based on the symmetrical short circuit current which is most easy to calculate among all types of circuit currents. But for the determination of relay settings, it is absolutely necessary to know fault current due to unsymmetrical condition too for which knowledge of symmetrical components is required.
Depending on the location, the type, the duration, and the system grounding, short circuits may lead to
• electromagnetic interference with conductors in the vicinity (disturbance of communication lines),
• stability problems,
• mechanical and thermal stress (i.e. damage of equipment, personal danger)
• danger for personnel
Typically, only 5% of the initial faults in a power system, are three phase faults with or without earth. Of the unbalanced faults, 80 % are line-earth and 15% are double line faults with or without earth and which can often deteriorate to 3 phase fault. Broken conductor faults account for the rest.
FAULTS IN TRANSMISSION LINES:
The following are the faults that occur in transmission lines:
Three Phase Fault
Single Line to Ground Fault
Line to Line Faults
Double Line to Ground Faults
Broken Conductor Fault
Single Line to Ground faults (L – G faults)
Line-to-ground faults are faults in which an overhead transmission line touches the ground because of wind, ice loading, or a falling tree limb. A majority of transmission-line faults are single line-to-ground faults.
The single line to ground fault can occur in any of the three phases. However, it is sufficient to analyze only one of the cases. Looking at the symmetry of the symmetrical component matrix, it is seen that the simplest to analyze would be the phase a.
Consider an L-G fault with zero fault impedance as shown in figure.
L-G fault on phase a
Since the fault impedance is zero, on occurrence of the the fault.
Va = 0 , Ib = 0 , Ic = 0
since load currents are neglected.
NEED FOR PROTECTION:
• As the generating stations are far away from the load centres they run over hundreds of kilometres. Hence, the chances of fault occurring in transmission lines are very high.
• Since faults can destabilize the power system they must be isolated immediately.
• Fault analysis is a very important issue in power system engineering in order that to clear faults quickly and restore power supply as soon as possible with minimum interruption.
FACTORS INFLUENCING LINE PROTECTION:
• Criticality of the line (in terms of load transfer and system stability)
• Fault clearing time requirements for system stability
• Line length
• System feeding the line
• Configuration of the line (the number of terminals, the physical construction of the line, the presence of parallel lines)
• Line loading
BLOCK DIAGRAM DESCRIPTION:
Power supply is not given directly to the transmission lines. It is given via a step down and step up transformers. The supply voltage is stepped down from 230V to 12V and stepped up to 230V. This is done to prevent the damage of fuse during a short circuit in the transmission lines.
The supply is then given to a potential transformer. The output of the potential transformer is rectified and is given as one of the inputs of the comparator.
A current transformer is connected line to transmission line. A current transformer is used because current changes due to short circuit faults. This current is then is then rectified and given as another input for comparator.
The comparator compares these two values and gives an output based on whether there is a fault or not.
The driver circuit is basically a microcontroller based circuit. The comparator output is given to the microcontroller. Based on the comparator output the microcontroller produces an output signal.
The relay used here is an electronically controlled relay. It is controlled by means of a microcontroller. This helps in changing the set value of the relay which is not possible in conventional relays.
When a fault occurs there is a change in the transmission line current. This is detected by the current transformer. The transformer output is then given to the comparator. Now the comparator will produce an output to indicate a fault. The microcontroller picks this up and produces a signal which trips the relay for the particular line. As we deal with low voltages such 230V we use relays directly. If used for higher voltages the relays are used to operate a circuit breaker.
CURRENT TRANSFORMER:
• A Current transformer (CT) is used for measurement of electric currents.
• When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments.
POTENTIAL TRANSFORMER:
• The potential transformer works along the same principle of other transformers.
• It is used to control the voltage. It converts voltages from high to low.
• It will take the thousands of volts behind power transmission systems and step the voltage down to something that meters can handle.
RELAY:
• A relay is an electrically operated switch.
• Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal.
• Relays have two circuits
Control circuit
Load circuit.
• The relay's most basic components are its coil, armature, and contacts.
• When the relay is put into some given circuit, the current from that circuit induces a magnetic field in the relay coil.
• The magnetic field in the coil then affects the armature in such a fashion that it causes the contacts to make or break the part of the circuit to which the relay output terminals are connected.