FUNDAMENTAL OF PROTECTIVE SYSTEM

FUNDAMENTAL OF PROTECTIVE SYSTEM

FUNCTIONS OF PROTECTIVE SYSTEM

The function of protective systsem is to disconnet the faulty part due to short circuit or to disconnect the circuit under abnormal conditions which might cause a great damage to the rest of the power system. For this purpose relays are usee. The relay senses the fault current through CT and energises the trip coil which causes the opening of CB of that particular system and the faulty section is disconnected. The remaining part of the power system is not affected.

The CBs are located in such a way that each generator, transformer, transmission line, feeders etc are completely disconnected from the rest of the system till fault is cleared. The other function of protective system is to provide indication, location of fault and type of fault. Thus fault clearing process becomes more easier.

The protective system should have the following essentials properties:

1.    Very quick operation; If the protective system operates quickly and fault is cleared in a shorter time, the components of the systems are protected from the probable damage and insulation provided remains unaffected.

2.    Reliability: The ability of the protective system to operate under the predestermined conditions is called its reliability. It is an essential requirement for the system to be protected.

3.    Discrimination: The protective system should have the ability to discriminate between faulty sections and healthy sections. It should immediately isolate the faulty section from the system without affecting the healthy section so that continuity of supply can be maintained.

4.    Provision for instruments: The protective system shall provide the proper place for the instruments like CTs, PTs etc on the panels easily accessible and seen for observations/inspections.

NORMAL AND ABNORMAL CONDITIONS (NORMAL/ABNORMAL OPERATION OF SYSTEM)

1.    Normal Conditions: It is the state of the system network in which below normal or normal current flows and voltage and frequency remains normal. The cudrrent is not diverted to the other non-intended paths or earth and it is cdonfined to pass in the devises path withoiut disturbance and the systsem works normally.

a)    In the normal condition the power producing device like 3-phase alternator should run at the proper speed by the prime mover with the “speed govering arrangaement” so that it produces a desired voltage at he desired frequency (my differe by + 1%).

b)    All the 3-phase voltage must be of equal in magnitude having phase displacement of 120⁰. This is shown in the figure.

c)    All the three live voltages V_RY, V_YB and V_BR are also balanced and symmetrical.

d)    The provision of protectsive system should be such that it should be most economical at the same time it should provide satisfactory protection.

e)    Due to load variations impedance drop (I_Z) varies which varies the voltage at the consumer’s end. It should not cross the limits of + 5% on load top changer shall be provided.

f)     Stand by generator is provided to add into the system in peak load hours.

                                               

2.    Abnormal Conditions: It is the defect in any part of the system which produce abnormalities like, over current, over voltage, disturbance in frequnecies and the elements are likely to be damaged and system is disturbed.

In the supply system, right from the generating station to consumer though the generators produced balance symmetrical 3-phase voltages. The operation becomes abnormal due to various faults in the system as mentioned below. Due to abnormalities following are the defects observed:

a) Voltage and current unbalanced    b) Over voltages, c) Reversal power

d) power swinging, e) Under frequency, f) Temperature rise, g) Unstability of system.

REASONS FOR FAULT OCCURRENCE

1.    Due to failure of insulation of pomponents and equipment parts. It may lead to short circuit.

2.    Due to mechanical injuries and over temperature conductor may break.

3.    Lightnings and switching surges may damage the transmission and distribution lines. Lines are also affectged by birds and objects, tgrees falling on them or due to wind pressure.

4.    Harmonics produced in rotating machines due to unbalanced currernts produce over heating.

5.    Drawbacks in system designing, lack of selection of good quality material, equipment machinery, protective elements etc. may acuse damage to the system.

6.    Due to mal-operation by operators.

7.    Earthquakes, lightning,storms, ice forming are also responsible to disturb the system.