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Corona, its Effects and its Prevention

Corona is a form of Partial discharge which is luminous and occasionally visible to the Naked Eye.

Corona are caused when the air around an energized conductor gets ionized, causing a discharge. Corona are caused when defects are present in the conductors such as jagged edges or cracks which cause high local electric field. A hissing sound can also be heard during the occurrence of Corona.

The Nitrogen molecules in the air get excited and result and cause ultraviolet radiation. Corona is sometimes visible as a hazy blue light around conductors, especially during the night.

The Effects of Corona

1. Corona is accompanied by the creation of ozone and nitrogen oxides. Nitrogen oxides may react with moisture in the air and form nitric acid a potentially corrosive substance.
2. Corona can cause damage to insulators used in High voltage applications.
3. Corona emissions are accompanied by the generation of radio waves which can interfere with commercial radio transmission. Sometimes, Corona can also result in noise which may disturb the neighborhood.
4. It can sometimes cause carbon deposits which may later result in arcing.

Controlled discharging of Inductive HV Equipment

High Voltage equipments need to be discharged prior to any maintenance work.  The discharge is usually done with an earthing rod after verifying that there is no voltage present on them with an non-contact voltage detector.

In the case of inductive equipment such as transformers and motors with high inductance value in their windings, a controlled discharge needs to be carried out.  Inductive components such as windings in transformers and motors have high inductance.  Sudden discharge of these windings will create a high discharge current and a rapid change in the flux which will result in a high voltage pulse (according to Lenz' Law). This can damage the winding insulation.

The controlled discharge is carried out with a discharge rod with a resistor in series.  The resistor used is a special non linear resistor which has a reverse temperature coefficient which means that as the temperature increases the resistance falls.  When the discharge is started, the resistance is high. As current flows through the resistor, the temperature of the resistor rises and its resistance falls.  Thus the current flow is increased.

The resistor ensures that the discharge is gradual.

what is prospective short circuit current?

Prospective Short Circuit Current refers to the maximum short circuit current which can flow in a circuit in the event of a fault such as a short circuit or an earth fault.  It is the maximum current which can flow in the system.  

All the components of the system such as the cables, bus bars and the measuring and protection systems should be rated with reference to the Prospective Short circuit current. The interrupting devices such as fuses and circuit breakers should have an interrupting capacity greater than the prospective short circuit current.   

Prospective Short Circuit Current depends on the resistances and reactances of the system.  The prospective Short Circuit Current can  be obtained by a Fault Level Calculation Study of the system.

Difference between salient pole and non-salient pole rotor used in synchronous generator.

synchronous machine can be characterized by the type of rotor used. Rotors used in Synchronous alternators can be classified into:

1)Salient and

2)Non-Salient Pole Rotors or cylindrical rotors

Salient pole rotors are used in application with speeds from 100 to 1500rpm. They are alternative known as "projected pole" type of rotors. The poles mounted on the rotor are made of lamination's made of steel. The poles are connected to the rotor shaft by means of dovetail joints. Each pole has a pole shoe around which the winding is wound. The salient pole rotor is generally used in applications where the prime mover is a hydel turbine or a combustion engine which have low or medium speeds. Salient pole rotors usually contain damper windings to prevent rotor oscillations during operation. Due to low speed, they are constructed with higher no. of pole (ranging from 8-24 or higher) to achieve system frequency so they can be said as higher pole machine. Almost all hydro power uses salient pole rotor synchronous generator.

                               Non-salient pole rotors are generally used in application which operate at higher speeds, 1500rpm and above. The prime movers in these applications are generally gas or steam turbines. These are sometimes known as "drum rotors". The rotor is a cylinder made of solid forged steel. The slots on which the windings are fixed are milled on the rotor. The number of poles is usually 2 or 4 in number. Since these rotors are cylindrical, the windage loss is reduced. The noise produced is also less. These rotors have higher axial length. These rotors do not need damper windings. Due to high speed, they are used with gas turbine and high speed steam turbine in nuclear power plant and thermal power plant.