Critical disruptive voltage and Visual critical voltage

 Critical disruptive voltage:

                 It is the minimum phase-neutral voltage at which corona occurs. Consider two conductors of radius r (cm) and spaced d (cm) apart.  If V is the phase-neutral potential, then potential gradient at the conductor surface is given by:

g =[V/ r loge (d/r)] volts / cm

           In order that corona is formed, the value of g must be made equal to the breakdown strength of air.  The breakdown strength of air at 76 cm pressure and temperature of 25ºC is 30 kV/cm (max) or 21·2 kV/cm (r.m.s.) and is denoted by go.

If VC is the phase-neutral potential required under these conditions, then,

go =[Vc/ r loge (d/r)] volts / cm

where go = breakdown strength of air at 76 cm of mercury and 25ºC = 30 kV/cm (max) or 21·2 kV/cm (r.m.s.)

∴ Critical disruptive voltage, Vc = go r loge d/r

                 The above expression for disruptive voltage is under standard conditions i.e., at 76 cm of Hg and 25ºC.  However, if these conditions vary, the air density also changes, thus altering the value of go.

             The value of go is directly proportional to air density.  Thus the breakdown strength of air at a barometric pressure of b (cm) of mercury and temperature of tºC becomes δ go where

δ = air density factor = 3.92b / 273 +  t

Under standard conditions, the value of δ = 1.

∴ Critical disruptive voltage ,V c = go δ r loge d/r

             Correction must also be made for the surface condition of the conductor.  This is accounted for by multiplying the above expression by irregularity factor mo.

∴ Critical disruptive voltage, Vc = mo go δ r loge d/r …. kV/phase

where,
mo = 1 for polished conductors
      = 0·98 to 0·92 for dirty conductors
      = 0·87 to 0·8 for stranded conductors

Visual critical voltage

                 It is the minimum phase-neutral voltage at which corona glow appears all along the line conductors.

                 It has been seen that in case of parallel conductors, the corona glow does not begin at the disruptive voltage Vc but at a higher voltage Vv , called visual critical voltage.

             The phase-neutral effective value of visual critical voltage is given by the following empirical formula :


Important terms due to Corona

where mv is another irregularity factor having a value of 1·0 for polished conductors and 0·72 to 0·82 for rough conductors.


Comments

Post a Comment

Popular posts from this blog

Nominal 'π' method

Image
                       In the nominal pi model of a  medium transmission line,  the series impedance of the line is concentrated at the centre and half of each capacitance is placed at the centre of the line. The nominal Pi model of the line is shown in the diagram below. In this circuit, By Ohm’s law By KCL at node a, Voltage at the sending end By ohm’s law Sending-end current is found by applying KCL at node c or Equations can be written in matrix form as Also, Hence, the ABCD constants for nominal pi-circuit model of a medium line are Phasor diagram of nominal pi model The phasor diagram of a nominal pi-circuit is shown in the figure below. It is also drawn for a lagging power factor of the load. In the phasor diagram the quantities shown are as follows; OA = V r  – receiving end voltage. It is taken as reference phasor. OB = I r  – load current lagging V r  by an angle ∅ r . BE = I ab  – current in receiving-end capacitance. It leads V r  by 90°. The line current I is the phasor su
Read more

BREAKDOWN MECHANISMS IN ZENER DIODE

Image
Avalanche breakdown:                        The avalanche breakdown takes place when both sides of the junction are lightly doped and consequently the depletion region is large. When the reverse bias voltage is increased the accelerated free electrons collide with the semiconductor atoms in the depletion region. Due to the collision the covalent bonds are broken and electron - hole pairs are generated. These new charge carriers so produced acquire energy from applied potential and in turn produce additional carriers. This forms a cumulative process called as avalanche multiplication. This avalanche multiplication causes the reverse current to increase rapidly. This leads to avalanche breakdown.  Zener breakdown:                         This Zener breakdown mechanism is fundamentally different from avalanche breakdown. Zener breakdown occurs when the electric field in the depletion region increases to point where it can break covalent bonds and generate electron hole pair. The electro
Read more

MESH ANALYSIS

Image
Introduction:                            In Mesh analysis, we will consider the currents flowing through each mesh. Hence, Mesh analysis is also called as  Mesh-current method .                          A branch is a path that joins two nodes and it contains a circuit element. If a branch belongs to only one mesh, then the branch current will be equal to mesh current.                          If a branch is common to two meshes, then the branch current will be equal to the sum (or difference) of two mesh currents, when they are in same (or opposite) direction. Procedure of Mesh Analysis:                          Follow these steps while solving any electrical network or circuit using Mesh analysis. Step 1 − Identify the meshes and label the mesh currents in either clockwise or anti-clockwise direction. Step 2 − Observe the amount of current that flows through each element in terms of mesh currents. Step 3 − Write mesh equations to all meshes. Mesh equation is obtained by applying KVL f
Read more