opened, the no-load speed would be 4 percent above synchronous speed. This governor would be defined as having 4 percent speed droop.
Paralleling an isochronous governor to an infinite bus would be impractical because any difference in speed setting would cause the generator load to change constantly. A speed setting slightly higher than the bus frequency would cause the engine to go to full-load position. Similarly, if the speed setting were slightly below synchronous speed, the engine would go to no- load position.
Setting speed droop on hydraulic governors is accomplished by adjusting the speed-droop knob located on the governor body. Setting the knob to position No. 5 does not mean 5 percent droop. Each of the settings on the knob represents a percentage of the total governor droop. If the governor has a maximum of 4 percent droop. the No. 5 position would be 50 percent of 4 percent droop. Setting speed droops on solid-state electronic governors is accomplished by placing the UNIT-PARALLEL switch in the
PARALLEL position. The governor speed droop is factory set, and no further adjustments are necessary.
ISOLATED BUS OPERATION. - In the following discussion, assume that one generator, called the master machine, is operating and that a second generator, called the slave machine, is being synchronized to the master machine. Governor con- trols on the master ernogenerator should be set to the ISOCHRONOUS or UNIT position. The governor setting on the slave generator must be set to the PARALLEL position.
NOTE: The hydraulic governor droop setting is an approximate value. Setting the knob to position No. 5 will allow you to parallel and load the generator set. Minor adjustments may be necessary to prevent load swings after the unit is operational.
When you are paralleling in the droop mode with other generator sets, the governor of only one set may be in the isochronous position; all others are in the droop position. The isochronous set (usually the largest capacity set) controls system frequency and immediately responds to system load changes. The droop generator sets carry only the load placed on them by the setting of their individual speed controls. Both voltage regulators should be set for parallel and automatic operation.
The slave machine is brought up to the desired frequency by operating the governor controls. It is preferable to have the frequency of the slave machine slightly higher than that of the master machine to assure that the slave machine will assume a small amount of load when the main circuit breaker is closed. Adjust the voltage controls on the slave machine until the voltage is identical to that of the master machine. Thus two of the requirements for synchronizing have been met: 'frequencies are equal and terminal voltages are equal.
There are several methods to check generator phase sequence. Some generator sets are equipped with phase sequence indicator lights and a selector switch labeled "GEN" and "BUS." Set the PHASE SEQUENCE SELECTOR SWITCH in the BUS position, and the "1-2-3" phase sequence indicating light should light. (The same light must light in either GEN or BUS position.) If "3-2-1" phase sequence is indicated, the slave machine has to be shut down, the load cables isolated, and two of the load cables interchanged at their connection to the load terminals.
Another method to verify correct phase sequence is by using the synchronizing lights. When the synchronizing switch is turned on, the synchronizing lights will start blinking. If the synchronizing lights blink on simultaneously and off simultaneously, the voltage sequences of the two machines are in phase. The frequency at which the synchronizing lights blink on and off together indicates the different frequency output between the two machines. Raise or lower the speed of the slave machine until the lights blink on together and offtogether at the slowest possible rate. If the synchronizing lights are alternately blinking (one on while the other is off), the voltage sequence of the two machines is not in phase. Correct this condition by interchanging any two of the three load cables connected to the slave machine.
Some of the portable generators being placed in the NMCB Table of Allowances (TOA) are equipped with a permissive paralleling relay. This relay, wired into the main breaker control circuit, prevents the operator from paralleling the generator until all three conditions have been met.
Now that all three paralleling requirements have been met, the slave machine can be paralleled and loaded.
If a synchroscope is used, adjust the frequency of the slave machine until the synchroscope pointer rotates clockwise slowly through the ZERO position (twelve o'clock). Close the main circuit breaker just before the pointer passes through the ZERO position. To parallel using synchronizing lights, wait until theContinue Reading