Type of Lightening Strokes

Type of Lightening Strokes

There are two main ways in which a lighting may strike the power system:

(a)Direct stroke.

(b) Indirect stroke.

(a)Direct stroke:

☛In case of direct lightening stroke, the charge from the cloud is directly discharged on the object below it.

☛After discharging, the current finds the path through the insulator to the pole to the ground.

☛The over voltage set up due to the lightening stroke has a very large magnitude and may cause flashover or puncture of line insulator.

The direct strokes can be classified as:

1) Stroke A

2) Stroke B

Direct lightening stroke

(i) Fig. shows the direct lightening stroke. It can be explained in two ways. Referring to first figure, suppose a overhead transmission line is exactly below the positively charged cloud and there is no more cloud near to this one, then the lightening discharge will occur from the cloud to the object (i.e. overhead line) below it. In this case the cloud will induce a charge of opposite sign on the tall object.

When the potential between the cloud and line exceed the breakdown value of air, the lightening discharge occurs between the cloud and the line. 

(ii) Referring to second figure if there are three clouds P, Q and R having positive, negative and positive charge respectively then the charge on the cloud Q is bound by the charge on cloud R.

☛If the cloud P shifts too near the cloud Q, then lightening discharged will occur between them and a charge on both these clouds disappears quickly.

☛The result is that charge on cloud R suddenly becomes free and it then discharges rapidly to earth, ignoring tall objects.

☛i.e.in second case the lightening discharge occurs on the overhead line as a

result of stroke A between the cloud P and

(b) Indirect stroke:

☛The charged cloud always creates the equal and opposite charges on the object below it by the electrostatic action.

☛Assume that a positively charged cloud is exactly above the overhead line.

☛Thus it creates the negative charge of equal magnitude on it. 

☛If the cloud discharges to earth or

other neighbouring cloud, the negative charge on the line is left behind.

☛The result is that negative charge rushes along the line is both directions in the form of travelling waves.

Indirect Lightening stroke

☛It may be worthwhile to mention here that majority of the surges in a transmission line are caused by he indirect lightening strokes.

Lightening Arresters

Lightening Arresters:

• Protection against direct lightening strokes is provided by using the earthing screen and ground wires. But these two apparatus fail to provide protection against travelling waves; which may reach the terminal apparatus.

• The lightening arresters or surge diverters provide protection against such surges.

(A) Construction of Simple L.A

• A lightening arrester or a surge diverter is a protective device, which diverts the high

voltage surges on the power system to the ground.

(B) Characteristics of non- linear resistance

• Fig. 4.39.2 (a) shows the basic construction of a lightening arrester. It mainly consists of a spark gap and a non-linear resistor in series with it. 

• One end of the surge diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded.

• The air gap between the spheres is so adjusted such that under normal line

voltage this gap does not produce any are. But a dangerously high voltage will breake

down the air insulation and form an arc.

• The non-linear resistance is the resistance which having the non-linear characteristics i.e. the magnitude of resistance decreases as the voltage (or current) increases and vice-versa. Fig. 4.39.2 (b) shows the volt /Amp characteristic of the non-linear resistor.

Working:

The action of the spark gap lightening arrester or surge diverter is explained

follows:

(i) Since the spark gap and non-linear resistance provides very high resistance to normal operating voltage, the lightening arrester conducts no current to earth or the gap is non- conducting. 

(ii) When the over voltage occurs on the system, the air insulation across the gap

breaks down and an arc is formed.

(iii) Now the non-linear resistance provides a low resistance path for the surge to the ground. In this way, the excess charge is diverted towards the earth and hence equipment is protected against failure.

(ii) It should be noted that while designing a lightening arrester that when the surge is over the are in gap should cease. Otherwise if the arc does not go out, the current would continue to flow through the resistor and both resistor and gap may be destroyed.

Rod Gap Arrester: Construction & Working

Rod Gap Arrester: Construction & Working

☛Rod gap arrester is a very simple type of surge diverter. Fig.  shows the construction of rod gap arrester. 

Construction Of Rod Gap Arrester

☛It mainly consists of two 1.5 cm rods, which are bent at right angles, and an air gap is provided in between them.

☛Upper rod is connected to the line circuit and the other rod is connected to earth.

☛While designing the rod gap arrester two important things should be taken in to 

account. Firstly the distance between gap and insulator must not be less than one third of the gap length so that the are may not reach the insulator and damage it. 

☛And secondly the gap length is so adjusted that breakdown should occur at 80% of spark- over voltage in order to avoid cascading of very steep wave fronts across the insulators.

Working Of Rod Gap Arrester

☛The operation of rod gap arrester is exactly same that of explained in previous section i.e. under normal operating conditions the gap remains non conducting.

☛On the occurrence of a high voltage surge on the line, the gap sparks over and the surge current is conducted to earth. Thus the excess charge on the line due to the surge is harmlessly diverted to earth.

Construction Of Rod Gap Arrester

       

Limitation Rod Gap Arrester

(i) Due to the absence of non-linear resistance the arc in the gap is maintained by the normal supply voltage after the surge is over. This leads to a short circuit on the system.

(ii) Due to excessive heat produced by the arc, rods may melt.

(iii) Performance of rod gap arrester is depends on the climatic conditions such as rain, humidity, temperature etc.

Horn Gap Arrester: Operation, Advantages & Limitations

Horn Gap Arrester: Operation, Advantages & Limitations

☛Fig. shows the construction details of horn gap arrester. 

☛It mainly consists of two horn shaped metal rod A and B separated by a small air gap and a series resister R and inductor L. 

☛The horn are so constructed that distance between them gradually increase towards the top. 

☛The horns are mounted on porcelain insulators.

☛One end of horn is connected to the line through a resistance R and choke coil L while the other end is effectively grounded. ☛Since the reactance of a coil is mainly depends on the frequency, it offers small

reactance at normal power frequency current but a very reactance at transient frequency current.

☛Thus the transients are blocked / bypassed before it enters the apparatus, The resistance R helps in limiting the fault current to a small value.

Operation of Horn Gap Arrester

☛It stated above that all gap type of arresters have same working principle.

☛The gap between the horns is so adjusted that normal supply voltage is not sufficient

to cause brake down of gap between the horns. 

☛Under normal condition supply voltage is insufficient to initiate the are between the gap and gap remains non-conducting. 

☛When there is a voltage surge, spark takes place across the small gap.

☛Due to magnetic effect of the arc, a force is exerted on the arc and causes it (arc) to travel up in the gap.

☛Fig. shows the details of horn gap arrester. And position 1, 2 and 3 shows that are moves progressively upward (points A and B).

☛At some position of the arc, the distance may be too great for the voltage to maintain the are. This causes the extinction of the arc.

circuit diagram of horn gap lighting arrester

Advantages of Horn Gap Arrester

(i) Since the magnetic effect draws the arc away causing increase in the length of the are. Thus the arc is self-extinguished. Therefore this type of arrester does not cause short circulating of the system after the surge is over as into the case of rod gap arresters.

(ii) Series resistance and reactance limits the fault current to a small value.

Limitations of Horn Gap Arrester

(i)Some external agency (like bird) may bridge the gap and make the device useless.

(ii) Atmospheric conditions may affect the setting of horn gap due to corrosion or pitting. This affects the performance of the arrester.

(iii) As compared to modern protective gears the operating  time of horn gap arrester is comparatively long about a second.

Street Lighting

Street Lighting

Steel tubular pole on the street are erected for lighting of streets.

✒Distance from road ends - 1.5 

✒Base of pole-Concrete plat-form below base plate

✒Earthing-All poles shall be perfectly eart here 

-Earth wire shall be carried through G.I. pipe for its protection.

✒Base plate-mild steel 30 cm x 30 cm x 1 cm.

✒Terminal box-contain fuse, nutral link and earth thi able.

✒Wires from terminal box to run through pipe for tube connection.

Street Light

                    

                       Fig : Street Light

Parks, roads, streets, bridges are to be provided with lighting schemes. The electricity supply authority is electricity board but maintenance and Poisson is in municipal corporation.

All these lighting schemes cone under the category "street lighting". 

Electric supply:- 3-phase 4 wire 415 volts AC supply.

- The load is balanced on R-N, Y-N, B.N. equally at 230 volts

- single phase.

Tarriff- It covers the energy consumed replacement may be from corporation /

municipal corporation.

1.Electric Light Sources :

• Incandescent lamps
• Mercury vapour lamps
• Sodium vapour lamps
• Fluorescent tubes
• Solar panels to Fluorescent tubes
• LEDS (Newly coming up pattern low wattage high illumination) 

2.Street Lighting Scheme and its Design:

Important points:

(i)Luminous intensity should be uniform.

(ii)Distance between two pole < 30 m to 35 m. 

(iii) All crossings and turns should be provided with street light points.

(iv) Height of lamps should be uniform.

(v) Height of illumination fittings should be based on their spacing and nature of load.

(vi) Street mains may be drawn along the distribution line.

(vii) The neutral conductor of the L.V. system should be used as the return conductor of street lighting system also.

(viii) Switch/time switch should be used for controlling street mains.

(ix) A fuse unit should be provided in each lamp circuit.

(x)Fuse unit shall be fitted on pole at a safe distance from ground level.

3.Basic Requirements of Street Lighting

(i) There should not be glace.

(ii)Objects shall be clearly visible upto 40 meters to the car vehicle drivers.

(iii)The road and the background to be illuminated against the objects on the road. Objects are viewed by contrast.

(iv) Location of lamps, types, surface of road, light distribution on road etc. are equally important.

(v) By less lamp power the distant objects on the road should be seen by the car-drivers.

(vi) Objects on the road get lesser incident light which is normally seen by the viewer and appear to be dark against illuminated road and background.

3.Locations of Lamps and their Advantages :

1.Central mounted

-two lamps back to back.

-Useful for very wide road

-Economical

2.Opposite mounting

-Superior to other methods

-Uniform illumination on road surface

-Used for very wide roads.

3.Stag arrangement

-Lamps on alternate columns, on the opposite side of central strip.

-Very suitable for 15 meter wide road on both sides of which there are foot-paths at extreme ends.

4.Single side mounted

-For bends with radius with less than 75X height of mounting.

-Not for straight roads.