INDUSTRIAL FREQUENCY TESTING TRANSFORMERS &TRANSFORMERS FOR OTHER TESTS

The design and construction of H.V. testing transformers depends on the service conditions, which will usually be such as to involve intermittent use, with H.V. discharges on the output side amounting to a partial short circuit. The mid – points of the transformers are connected to the tanks, and each and of the winding is suitably tapped and insulated to permit of one end being used for magnetizing and primary load current, the other end for the magnetizing and load current of the next transformer. All transformer tanks have to be insulated from earth but the winding insulation has only to suffice for one – half of the voltage of the unit to the core.

The cascade connection simplifies the insulation of the transformers but the small portions of the windings which carry both magnetizing and load currents on the input sides give considerable reactance, and compensating windings may be necessary to maintain the voltage on load. When a H.V. transformer is on open circuit, the secondary (H.V.) side may have sufficient capacitance to cause a charging current to circulate, the counterpart of which has to be supplied to the primary together with the magnetizing current. H.V. transformers are usually of the core type, since the insulation of the windings presents a simpler mechanical and electrostatic problem, and bakelized paper cylinders are used to a considerable extent to build up the concentric windings.

In the three further H.V. supplies for testing listed the transformer is an essential feature, but in these cases its output is rectified, usually by means of H.V. thermionic diodes. The production of high direct voltages which show half and full wave rectification, the capacitors being employed to maintain the output voltage. Since the rectifiers are at high voltage to earth, their cathode supplies must be insulated, or H.V. filament transformers used with secondaries well insulated from the primaries. A similar arrangement is necessary for the surge generator in which a bank of H.V. capacitors is charged through a rectifier from the secondary of a H.V. transformer. When the voltage across the capacitors has risen to a pre – set value, the trigger gaps spark over together, connecting the capacitors momentarily in series and raising the voltage of the output terminals to a high value. If there is a circuit across the output terminals, the energy of the capacitors is released therein. A typical test specimen would be an artificial overhead line, or cable.

For the generation of high–frequency voltages the transformer charges a capacitor through a resistance and rectifier. When the voltage developed across the capacitor is sufficient, the trigger gap breaks down, and connects the primary of a Tesla Coil in series with the capacitor, which then initiates a H.V. oscillatory resonant discharge. The oscillations are transformed up in the secondary of the Test coil. The latter may comprise two concentric coreless coils of spaced bar conductor. The frequency developed coreless coils of spaced bar – conductor. The frequency developed depends upon the inductance and capacitance of the primary side, the degree of coupling, and the load on the secondary of the Tesla transformer. The Buchholz system is applicable to oil – immersed transformers the great majority and depends on the fact that transformer breakdown are always preceded by the more or less violent generation of gas. A broken joint for example produces a local are and vaporizes the oil in the vicinity.