Synchronizing EquipmentThis can be in the form of a synchronizing meter or a system of lamps connected across the a.c. generators to beparalleled. The synchronizing meter gives a direct indication when the machines are in synchronism. The lamps areconnected across like phases, i.e. U-U. V-V, W-W, such that the machines are in phase when the lights are dimmed. Itis important to rate the lamps to at least twice the machine voltage and in most instances it would be necessary toconnect two or three in series.Protection DevicesIn addition to essential protection devices, i.e. overload circuit breaker, fuses, etc. it is recommended that the breakershave a shunt release coil working in conjunction with a reverse power relay trip. This ensures that, should one of the setsmalfunction for any reason, i.e. engine shutdown, voltage drop, etc. the power transferred from the other set, which isnow trying to 'motor' the failed plant, will then de-energize the contact breaker and disconnect the sets.Setting Up Procedure for Parallel OperationThe droop equipment should be tested and set correctly and the machines can be set for synchronization as follows:1.The speed of each set must be adjusted to the nominal frequency of the system, i.e. 50 Hz or 60 Hz.2.The voltage of each machine can now be set identically on the hand voltage trimmers or A.V.R. range control.3.The machine should only be paralleled when the difference in frequency between the two sets is small enough toenable the breaker to be closed when the in-phase condition is observed. This is achieved by adjusting the speed ofthe incoming set to match the set on the bus-bars and the easiest method of speed adjustment is with a governormotor control and a Raise/Lower speed control button.Care must be taken to ensure that the machines are exactly in phase before paralleling is attempted, as theforces set up by out of phase paralleling can create severe mechanical and electrical stresses in the sets.4.As load is applied any discrepancy between the kW meters can be eliminated by slight adjustment of the speed onone of the machines.5.With the kW meters sharing load correctly, any discrepancy between the ammeters can be eliminated with slightadjustment of the voltage on one of the machines.NOTE: Current discrepancies are due to circulating currents which flow between the sets when there is a difference inexcitation levels between the a.c. generators. High circulating currents may be due to a reversal in one of thedroop transformers or incorrect adjustment of the droop setting on the burden resistor or choke. In general,when paralleling identical machines, the amount of droop should be approximately equal on all sets.38
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
