Circuit Breaker Analyzer:
Short Circuit Generator: This is of a special design having very low reactance in order to give the maximum short-circuit output of the Circuit Breaker Analyzer. The leakage reactance is reduced by reducing the depth of slots and the length of coil ends. The terminals are brought to a board where different connections can be made. Each winding in a phase is divided in two halves which can be connected either in series or parallel. The three windings in turn can be connected in star or delta to give different terminal voltages.
Due to heavy electrodynamic forces involved the generator foundation is specially designed and is isolated by means of materials like cork, neoprene and thermocole in order to prevent the vibrations being transmitted to the other parts of the building. The windings are also specially braced and made rugged. The short-circuit generator is provided with closed circuit air cooling. The cooling Circuit Breaker Analyzer consists of axial flow fans and air/water coolers. The generator is driven by a three-phase induction motor connected through a resilient shaft. The generator is equipped with a built-in flywheel which provides the kinetic energy during short circuit and speed regulation of the set.
Impulse Excitation: Normal excitation is supplied by the main exciter coupled to the test set, although the main generator is equipped with a damper winding, impulse excitation or super excitation is provided to counteract the demagnetizing effect of armature reaction. The short-circuit currents which are at lagging power factor have demagnetizing effect. This results in a reduction of total field, hence in reduced induced emf. As a result recovery voltage is less than the voltage before short circuit. It is overcome by shorting out a resistor in the generator field circuit, a moment before the short circuit, resulting in a surge of field current 8 to 10 times its normal value. This takes care of the demagnetizing effect of short-circuit current and gives desired recovery voltage.
Pilot Generator: This is a small three-phase synchronous generator directly coupled to the main shaft of the short-circuit generator and synchronized in phase with the latter. Any present voltage is maintained constant by an automatic voltage regulator during short-circuit tests. This dependable voltage is necessary to supply control power to the sequence timer, electromagnetic oscillograph and various other actuating circuits for conducting the test.
Short Circuit Transformers: These transformers are designed to withstand repeated short circuit’s and their windings are often arranged in sections for voltage adjustment in series and parallel combinations. The leakage reactance of the short-circuit transformer is kept low. For stepping down the voltage to lower values a three-phase transformer is normally used. For voltages higher than the generated voltage the normal practice is to use banks of single-phase transformers. Depending on the type of test to be carried out in the test cells, different types and ratings of transformers are connected in different cells, e.g. a high voltage testing transformer in one and a low voltage high current transformer in the other.
Resistors and Reactors: For regulation of the short-circuit test current, three-phase banks of resistor and reactor are used. The reactor being used to adjust the magnitude and the resistor to control the rate of decay of the d. c: component of current. The short-circuit power factor is also controlled by this means. There are a number of coils per phase and by connecting them in series parallel combinations many variations can be obtained.
Master Circuit Breaker: It has been provided mainly as a backup circuit breaker. If the object under test fails the current will be interrupted by the master breaker. This is normally an air-blast type, of capacity more than the breakers under test. This is set to open at a predetermined time after the initiation of the short circuit and must be capable of clearing faults up to the highest level of each generator or other power source and have a substantial margin of safety.
Make Switch: The short circuit is applied by this switch, which makes an exceptionally high, short-circuit current surge from the generator at a predetermined point of the voltage wave. At the moment of making, the backup circuit breaker and the test Circuit Breaker Analyzer are already closed. As such on closure if there is even a slight contact rebound the contacts might get burnt or even welded together. In order to ensure, that the contact separation is minimum on closure, a high air pressure is maintained in the chamber. The closing speed is also so high that the contacts are fully closed before the short-circuit current reaches its peak value.
These are used for two purposes.
- Capacitive breaking, which normally occurs in case of line charging duty.
- Controlling the rate of rise of restriking voltage given by
In synthetic testing and other indirect tests, capacitors are an important element of the test circuit. These are single-phase banks and can be connected in series or parallel as desired, both individually and in any combination of the three.
Test Cells: Covered bays are provided for testing Circuit Breaker Analyzer. These bays are box shaped structures of reinforced concrete with an open front facing the control and observation rooms. Separate test bays are provided for testing LV; HV; and EHV circuit breakers. The test cells are supplied with compressed air and oil purification system to facilitate testing of air-blast circuit breakers and oil circuit breakers. Each test cell has got an end box where control and measuring cables are terminated.
Test Control Room: It is situated nearly at a distance of 20m from test cells. It is provided with a control desk for remote control operation of the excitation of the short-circuit generators, etc. there are also an oscillograph, distribution panel, an electromagnetic oscillograph; a CRO, an electric sequence switch and an electronic sequence switch. The control room is designed to provide facilities for the observation of tests by both the station staff and visitors. For this purpose observation slits covered with thick perspex sheet are provided in the otherwise closed wall of the control room building facing the test cells.
Safety and Signalling System: A fool proof interlocking system is provided to ensure the safety of the working personnel, as enormous electrodynamic forces are brought into play during short-circuit testing. By this system it is impossible to excite the machines, if any of the doors leading to machine hall basement reactor room, etc. remain open. A system of audiovisual alarms are also provided as a warning before short-circuit tests.