Corona

An electric field modifies the random motion of the ions normally present in air. The components of motion produced by the electric field are such that negative ions are impelled toward the positive electrode and the positive ions toward the negative electrode. As the electric field intensity is increased the velocity of the ions increases accordingly. A field intensity of 3,000 kv per m will accelerate some of the ions in air, at atmospheric pressure, to velocities such that collisions between them and neutral molecules release an electron from the neutral molecule producing a pair of ions with opposite charges. This phenomenon is known as ionization by collision. Corona is thus produced. When the electric field intensity at the surface of the conductor is increased beyond the critical value, the formation of corona is accompanied by a hissing sound that is most intense in regions where the conductor has sharp points or corners. At those regions the electric field intensity is the greatest. Corona accompanied by sound is known as audible corona. In a darkened room, corona formation produces a luminous glow around a conductor with streamers emanating from corners and points. If the electric field intensity is raised to a sufficiently high value the cumulative effect of the collisions of ions with neutral molecules will produce an ion avalanche and electrical breakdown. If the field is uniform, as between plane parallel-plate electrodes, complete breakdown occurs suddenly with the formation of corona.

Table 2-4. Relative Dielectric Constants And Dissipation Factors
Solids	f [cps]	102	103	104	105	106	107	108	3x108	3x109	1010
Porcelain dry process	ε'/ε0	5.50	5.36	5.23	5.14	5.08	5.04	5.04	5.02		4.74
	tan δ	0.022	0.014	0.011	0.009	0.008	0.007	0.008	0.010		0.016
Teflon	ε'/ ε0	2.1	2.1	2.1	2.1	2.1	2.1	2.1	2.1	2.1	2.08
	tan δ	0.0005	0.0003	0.0003	0.0003	0.0002	0.0002	0.0002	0.00015	0.00015	0.0006
Liquids	f [cps]	102	103	104	105	106	107	108	3x108	3x109	1010
Cable oil	ε'/ε0	2.25	2.25	2.25	2.25				2.24	2.22	2.22
	tan δ	0.0003	0.00004	0.00004	0.0001				0.0039	0.0018	0.0022
Pyranol	ε'/ε0	4.42	4.40	4.40	4.40	4.40	4.40	4.08	3.19	2.84	2.62
	tan δ	0.0036	0.0003	0.0003	0.00036	0.0025	0.0260	0.15	0.15	0.12	0.074

Corona is undesirable in electrical equipment. In the case of high-voltage aerial transmission lines, the corona increases the losses and thus lowers the efficiency. In addition, it produces radio interference. Large conductors are used in high-voltage aerial lines to prevent the formation of corona. Some of these conductors are hollow and some are made of aluminum strands reinforced by a core of one or more steel strands. The insulation in high-voltage equipment such as underground cable and capacitors is thoroughly impregnated to prevent voids in which corona can form. Transformers in the higher voltage ratings are oil-insulated. The oil serves two purposes, one of which is to increase the dielectric strength, and the other to reduce the thermal resistance, thus making for good heat transfer. Another method of suppressing corona is to operate the insulation or dielectric under hydrostatic pressure. Some high voltage cables that use impregnated paper insulation are provided with channels where nitrogen gas is maintained under pressure.

Corona shortens the life of dielectric materials. In air, corona produces ozone O₃ and oxides of nitrogen. Although ozone is unstable, it is a strong oxidizing agent and attacks most organic materials. The oxides of nitrogen produce nitric and nitrous acids in the presence of moisture; these acids embrittle cellulose and corrode metal.

When corona starts in a dielectric, the dissipation factor increases with increasing electric field intensity. Dielectrics used in commercial insulation exhibit a practically constant dissipation factor at a given temperature for voltages well beyond the normal operating range as long as there are no voids. Corona loss increases the conduction current I tan δ in Fig. 2-25.