31. Why is the MMF method of estimating the voltage regulation considered as the optimistic method?
Compared to the EMF method, MMF method, involves more number of complex calculation steps. Further the OCC is referred twice and SCC is referred once while predetermining the voltage regulation for each load condition. Reference of OCC takes care of saturation effect. As this method require more effort, the final result is very close to the actual value. Hence this method is called optimistic method.
32. State the condition to be satisfied before connecting two alternators in parallel
The following are the three conditions to be satisfied by synchronizing the additional Alternator with the existing one or the common bus-bars.
• The terminal voltage magnitude of the incoming Alternator must be made equal to the existing Alternator or the bus-bar voltage magnitude.
• The phase sequence of the incoming Alternator voltage must be similar to the bus-bar voltage.
• The frequency of the incoming Alternator voltage must be the same as the bus-bar voltage.
33. How do the synchronizing lamps indicate the correctness of phase sequence between existing and incoming Alternators?
The correctness of the phase sequence can be checked by looking at the three sets of lamps connected across the 3-pole of the synchronizing switch. If the lamps grow bright and dark in unison it is an indication of the correctness of the phase sequence. If on the other hand, they become bright and dark one after the other, connections to any two machine terminals have to be interchanged after shutting down the machine.
34. What are the advantages and disadvantages of three dark lamps method of synchronizing?
Advantages:
• The synchronous switch using lamps is inexpensive
• Checking for correctness of the phase sequence can be obtained in a simple manner which is essential especially when the Alternator is connected for the first time or for fresh operation after disconnection
Disadvantages:
• The rate of flickering of the lamps only indicates the frequency difference between the bus-bar and the incoming Alternator. The frequency of the incoming Alternator in relation to the bus-bar frequency is not available.
35. How synchronoscope is used for synchronizing Alternators?
Synchronoscope can be used for permanently connected Alternators where the correctness of phase sequence is already checked by other means. Synchronoscope is capable of rotating in both directions. The rate of rotation of the pointer indicates the amount of frequency difference between the Alternators. The direction of rotation indicates whether incoming Alternator frequency is higher or lower than the existing Alternator. The TPST switch is closed to synchronise the incoming Alternator when the pointer faces the top thick line marking.
36. Why synchronous generators are to be constructed with more synchronous reactance and negligible resistance?
The presence of more resistance in the Synchronous generators will resist or oppose their synchronous operation. More reactance in the generators can cause good reaction between the two and help the generators to remain in
synchronism in spite of any disturbance occurring in any one of the generators.
37. List the factors that affect the load sharing in parallel operating generators?
The total active and reactive power delivered to the load, connected across the common bus-bars, are shared among Synchronous generators, operating in parallel, based on the following three factors
• Prime-mover characteristic/input
• Excitation level and
• Percentage synchronous impedance and its R/X ratio
38. How does the change in prime mover input affect the load sharing?
An increase in prime-mover input to a particular generator causes the active- power shared by it to increase and a corresponding decrease in active-power shared by other generators. The change in reactive power sharing is less appreciable. The frequency of the bus-bar voltage will also subjected to slight increase in value.
39. How does change in excitation affects the load sharing?
The decrease in excitation in one generator causes the reactive power shared by it to decrease and a corresponding increase in reactive-power shared by
other generators. The change in active-power sharing is less appreciable. There will be a slight decrease in terminal voltage magnitude also.
40. What steps are to be taken before disconnecting one Alternator from parallel operation?
The following steps are to be taken before disconnecting one Alternator from parallel operation
• The prime-mover input of the outgoing generator has to be decreased and that of other generators has to be increased and by this the entire active-power delivered by the outgoing generator is transferred to other generators.
• The excitation of the outgoing generator has to be decreased and that of other generators have to be increased and by this the entire reactive-power delivered by the outgoing
generator is transferred to other generators.
• After ensuring the current delivered by the outgoing generator is zero, it has to be disconnected from parallel operation.
Compared to the EMF method, MMF method, involves more number of complex calculation steps. Further the OCC is referred twice and SCC is referred once while predetermining the voltage regulation for each load condition. Reference of OCC takes care of saturation effect. As this method require more effort, the final result is very close to the actual value. Hence this method is called optimistic method.
32. State the condition to be satisfied before connecting two alternators in parallel
The following are the three conditions to be satisfied by synchronizing the additional Alternator with the existing one or the common bus-bars.
• The terminal voltage magnitude of the incoming Alternator must be made equal to the existing Alternator or the bus-bar voltage magnitude.
• The phase sequence of the incoming Alternator voltage must be similar to the bus-bar voltage.
• The frequency of the incoming Alternator voltage must be the same as the bus-bar voltage.
33. How do the synchronizing lamps indicate the correctness of phase sequence between existing and incoming Alternators?
The correctness of the phase sequence can be checked by looking at the three sets of lamps connected across the 3-pole of the synchronizing switch. If the lamps grow bright and dark in unison it is an indication of the correctness of the phase sequence. If on the other hand, they become bright and dark one after the other, connections to any two machine terminals have to be interchanged after shutting down the machine.
34. What are the advantages and disadvantages of three dark lamps method of synchronizing?
Advantages:
• The synchronous switch using lamps is inexpensive
• Checking for correctness of the phase sequence can be obtained in a simple manner which is essential especially when the Alternator is connected for the first time or for fresh operation after disconnection
Disadvantages:
• The rate of flickering of the lamps only indicates the frequency difference between the bus-bar and the incoming Alternator. The frequency of the incoming Alternator in relation to the bus-bar frequency is not available.
35. How synchronoscope is used for synchronizing Alternators?
Synchronoscope can be used for permanently connected Alternators where the correctness of phase sequence is already checked by other means. Synchronoscope is capable of rotating in both directions. The rate of rotation of the pointer indicates the amount of frequency difference between the Alternators. The direction of rotation indicates whether incoming Alternator frequency is higher or lower than the existing Alternator. The TPST switch is closed to synchronise the incoming Alternator when the pointer faces the top thick line marking.
36. Why synchronous generators are to be constructed with more synchronous reactance and negligible resistance?
The presence of more resistance in the Synchronous generators will resist or oppose their synchronous operation. More reactance in the generators can cause good reaction between the two and help the generators to remain in
synchronism in spite of any disturbance occurring in any one of the generators.
37. List the factors that affect the load sharing in parallel operating generators?
The total active and reactive power delivered to the load, connected across the common bus-bars, are shared among Synchronous generators, operating in parallel, based on the following three factors
• Prime-mover characteristic/input
• Excitation level and
• Percentage synchronous impedance and its R/X ratio
38. How does the change in prime mover input affect the load sharing?
An increase in prime-mover input to a particular generator causes the active- power shared by it to increase and a corresponding decrease in active-power shared by other generators. The change in reactive power sharing is less appreciable. The frequency of the bus-bar voltage will also subjected to slight increase in value.
39. How does change in excitation affects the load sharing?
The decrease in excitation in one generator causes the reactive power shared by it to decrease and a corresponding increase in reactive-power shared by
other generators. The change in active-power sharing is less appreciable. There will be a slight decrease in terminal voltage magnitude also.
40. What steps are to be taken before disconnecting one Alternator from parallel operation?
The following steps are to be taken before disconnecting one Alternator from parallel operation
• The prime-mover input of the outgoing generator has to be decreased and that of other generators has to be increased and by this the entire active-power delivered by the outgoing generator is transferred to other generators.
• The excitation of the outgoing generator has to be decreased and that of other generators have to be increased and by this the entire reactive-power delivered by the outgoing
generator is transferred to other generators.
• After ensuring the current delivered by the outgoing generator is zero, it has to be disconnected from parallel operation.
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