SESSION:14

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1)       What are the different electrical clearances in Substation?

        Ground Clearance: This is the minimum distance between any live conductor and the earth or             ground.

        Phase Clearance: It is the clearance between the different phases in a circuit or same phases in             different circuits

       Sectional Clearance: This is the minimum clearance between the live conductor and the limits of         work, where special clearance is related to the safety of the operating personnel. The limits of             work section may be ground or the platform on which the person is working.

2)       Why EHV busbars are hallowed and circular?

To avoid corona effect.

3)       What is the first equipment of the substation connected to the incoming transmission line?

Lightning arrestor is the first equipment of the substation connecting the incoming transmission lines. It protects other substation equipment from lightning and switching surges by suppressing them to ground. Lightning arrestors also placed in the substation between both the ends of major equipment such as power transformers for protection. 

4)       Why SF6 is used in Circuit Breaker? What are their properties?

Sulfur hexafluoride gas (SF6) gas properties:

·         Heavy, chemically inert, non toxic

·         No poisonous effect on the human body but decomposition products are poisonous

·         Color less and odor less

·         It is gaseous at normal room temperature and pressure

·         Density is about 6.6g/l at 20oC (5 times denser than air)

·         Critical temperature is at 45.6oC and can be liquefied by compression

·         Very good insulant with high dielectric strength

·         SF6 gas is electo-negitive ( tends to attract the free electrons and has the arc quenching property). Because of this main reason SF6 gas is used for arc quenching and insulation medium in circuit breakers.

·         The gas is highly stable

·         Unlike solid insulation materials, electrical breakdown of sf6 gas does not result in permanent deterioration of its properties

·         Decomposition occurs on the exposure to the electric arc. (Disassociation products will be SF2 and SF4 lower order fluorides)  

5)       What do you mean by Creepage distance in Insulator?

The creepage distance for insulators is the shortest distance along the insulator surface between the metal parts at each end of the insulator. Creepage distance can also be refer as leakage distance for insulators.
Insulators in substation are provided to avoid any leakage current from live electrical conductors to flow to the earth through supports. The atmospheric dust sticks to the insulator surface forming a conducting layer. The leakage current flows from the live conductor to the earth through such surface layers. The leakage properties (creepage properties) of an insulator s in substation are characterized by the length of the leakage path. While designing the insulator sheds, the leakage distance for insulators requirement should be satisfied. The requirement of the leakage distance or creepage distance for insulators depends on the

·      Rated phase to ground voltage

·      Degree of atmospheric pollution

If the surface of the insulator is clean, smooth and well gazed the dust particles tend to fall down. When an a.c voltage is applied to the apparatus, the dust particles have a tendency to align to the direction of electrical field lines on the surface of the insulator. If the surface of the insulator is rough and moist this alignment will be faster. The continuous application of voltage causes slow alignment of these particles. To prevent a continuous conducting track, the insulator should have sufficient ceepage distance or leakage distance.

During power frequency withstand test of an unclean insulator, the flash-over occurs along the dirty surface of the insulator. In case of internal gas filled or oil filled apparatus, the internal surface should also be free from moisture and dust. Otherwise internal flash over can occur along the surface by tracking.

6)       What is meant by protective angle? Give its value for reliable operation?

Protective angle is the vertical angle through the ground wire axis and the line passing from the ground wire axis to the outermost phase conductor. Its value for reliable protection is taken equal to 20-30 degrees.

7)       What is meant by impulse ratio of any lightning arrester?

Impulse ratio of any lightning arrester is the ratio of breakdown voltage of the wave of special duration to breakdown voltage of a 50Hz wave.

8)       What is meant by voltage surge?

Voltage surge is sudden excessive rise in voltage that may be dangerous to the electrical equipment of an installation. Voltage surge cause damage to insulation of the system. Voltage surges are caused because of lightning, switching on and off the load at high voltages.

9)       How standard lightning impulse wave is represented?

Standard Lighting impulse wave is represented by 1.2/50 micro sec wave.

10)    What is meant by Time Graded Protection?

Time Graded protection is a scheme of over current protection, in which the discrimination is incorporated i.e., the time setting of the relays is so graded that in the event of the fault, the smallest part of the system is disconnected.