In solid dielectrics, highly purified and free of imperfections, the breakdown strength is high, of the order of 10 MV/cm. The highest breakdown strength obtained under carefully controlled conditions is known as the “intrinsic strength” of the dielectric. Dielectrics usually fail at stresses well below the intrinsic strength due usually
Year: 2014
Almost no complete electrical insulation consists of one insulating phase. Usually more than one insulating material will be involved, either in series, parallel or both. The simplest form of composite insulation system consists of 2 layers of the same material. In this case advantage is taken of the fact that
Since no insulant is completely free of ions, a leakage current will flow when an electric field is applied. The ions may arise from dissociation of impurities or from slight ionisations of the insulating material itself. When these ions reach the electrodes, reactions occur in accordance with Faraday’s law of
Surface flashover Surface flashover is a breakdown of the medium in which the solid is immersed. The role of the solid dielectric is only to distort the field so that the electric strength of the gas is exceeded. If a piece of solid insulation is inserted in a gas so
(a) Removal of dust Small dust particles can become charged and cause local stresses which can initiate breakdown. They can also coalesce to form conducting bridges between electrodes. Careful filtration can remove dust particles greater in size than 1 m. The strength of the liquid then increases and greater stability
When a difference of potential is applied to a pair of electrodes immersed in an insulating liquid, a small conduction current is first observed. If the voltage is raised continuously, at a critical voltage a spark passes between the electrodes. The passage of a spark through a liquid involves the
Power systems equipment must withstand not only the rated voltage (Vm), which corresponds to the highest voltage of a particular system, but also over voltages. Accordingly, it is necessary to test high voltages. Equipment during its development stage and prior to commissioning. The magnitude and type of test voltage varies
• The slow speed salient pole alternators are ventilated by the fan action of the salient poles which provide circulating air. • Cylindrical rotor alternators are usually long, and the problem of air flow requires very special attention. • The cooling medium, air or hydrogen is cooled by passing over
Synchronous machines are principally used as alternating current (AC) generators. They supply the electric power used by all sectors of modern societies: industrial, commercial, agricultural, and domestic. Synchronous generators usually operate together (or in parallel), forming a large power system supplying electrical energy to the loads or consumers. Synchronous generators
Fig. (3.6) shows the voltage build up in a series generator. Here R1, R2 etc. represent the total circuit resistance (load resistance and field winding resistance). If the total circuit resistance is R1, then series generator will build up a voltage OL. The line OC is tangent to O.C.C. and
