No LoadReal Load Inductive Load
Capacitive Load
Parallel operation
Requirements for Paralleling
Production of A Voltage Error UNIT 14 LAMPS BRIGHT METHOD
AIM OF THE UNIT:- to understand lamps bright method
TASKS 1 Do your best to answer the brainstorming question. 2 Read the text for general understanding. 3 Make up questions to the text. 4 Find the sentences with the new words in the text. Give the Kazakh or Russian equivalents of the words. 5 Write sentences with the new vocabulary. 6 Make up exercises as in the UNIT 2 (Master`s degree students individual work with the teacher) page 210 (exercises for better remembering the topic). 7 Speak on the topic. Given schemes will help you to remember and understand the topic. 8 Find more information about the topic and make up a project work on the topic.
1 What is lamps bright method? For this method we are looking at the phase relationship between the generator A phase and the tie busbar B phase. A second lamp is also shown connected between the generator B phase and the tie busbar A phase. The ideal conditions for connecting the two supplies in parallel are with the lamps bright. LOCKING IN PARALLEL:When a.c. generators are connected in parallel, then they are said to be 'locked' together, let’s see how. When two generators are connected in parallel they are rarely in perfect synchronization at the instant the GCB is closed. An error voltage is produced which causes a circulating current to flow between the generators. The generator that is lagging in phase tends to be driven as a motor while the generator that is leading in phase has a retarding torque, both of these effects are caused by the circulating current. The motor and retarding effects very quickly pull both generators into synchronization with each other. This situation also applies to multiple generators operating in parallel. Normal Parallel Connection:Prior to starting the engines we can expect either the external power or the APU generator to be supplying the busbars via the BTBs. Now, for the next stage in the operation some aircraft are automatic while others are manual. With the automatic system the generator switches are selected to the ON position prior to starting the engines. As the engine speed approaches ground idle the generator's voltage and frequency stabilize and it is ready to come on line. The external power contactor or the APU contactor is opened and the GCB closes automatically. The manual system is similar but normally a flight engineer will select the GCB closed after the engine has started, the external power contactor or the APU contactor opens and the GCB closes. The second generator is brought on line via its GCB when its GCU is satisfied that it is safe to connect in parallel. The other generators are brought on line in a similar manner. Crash Paralleling:To connect generators in parallel when they are out of phase can cause damage to the CSD. The flight engineer or pilot can only in extreme emergency conditions connect the generators in parallel when they are not synchronized. This is achieved by first opening the BTB, closing the GCB and then reclosing the BTB, we say that the system has been 'crashed paralleled'. Remember that our 'auto-parallel' circuit only controls the GCB and not the BTB. CSD Shock Loads: In order that two generators may operate in parallel their voltages must be in phase with each other with reference to the load. But the two voltages are 180 degrees out of phase with respect to each other. Open the switch and you can see that the generators' voltages must oppose each other. If an error voltage is produced, and a circulating current is produced between the generators, it is the sudden application of this circulating current that can damage or even shear the drive of the CSD.
|
