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Key principles of operation when paralleling

  •  

    When generator sets operate in parallel, the engine speed governor of each generator set determines the proportional sharing of the total active power requirements (kW) of the system. 

    For two systems to be paralleled, each must have the following matching characteristics for correct synchronization: Phase Number: Both systems must be either single phase or three phase for correct synchronization.

    Phase Rotation: All three phases of each three phase system must be matched to avoid power surges and excessive electrical or mechanical stress. Matching just one phase is not enough. (See Graphic One and Two)

    Phases A, B and C are 120° apart and may rotate in either A-B-C or A-C-B sequences. A rotation meter verifies if sequences match. Frequency: The standard frequency is 60Hz in the US and 50Hz in some countries. The frequencies of the two systems must be the same for correct synchronization. 60 cycles per second cannot be synchronized with 50 cycles per second.

    Voltage: Each system being matched must be configured for the same voltage.

    Voltage Phase Angle: Phase angle matching requires that the wave forms rise and fall in together with no angle difference. The potential difference between the phases being matched must be zero. With a synchronous generator, matching is achieved by speed control.

     

    The kW load sharing is achieved by increasing or decreasing fuel to the systems’ engines.

     

    As the fuel to the engine of one generator set in a group is increased it will not lead to an increase in speed and hence frequency (as it would if it were operating alone) but it will lead to an increase in the proportion of the total kW load that it will deliver.

     

    As the fuel to the engine of one generator set in a group is decreased it will not lead to a decrease in speed and hence frequency (as it would if it were operating alone) but it will lead to a decrease in the proportion of the total kW load that it will deliver.

     

    The control system of the generator sets (via the engine speed control system) monitors and controls the sharing of the total kW load in proportion to the relative rating of the engines on the systems’ generator sets.

     

    1. The prime mover of the generator has to bring the speed of the shaft close to the rated speed of the generator.
    2. The excitation of the generator has to be increased to give a no-load output voltage as close as possible to the existing bus voltage

     

    We want to create a phasor rotation for generator 2 output similar to the bus voltage phasor.

     

    1. Observe the lights which are connected across the switches: they should beat, first get brighter and then dim as the phasors for generator and bus respectively shift. If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat out of phase, one pair of phases should be
    2. Adjust now the prime mover to slowly increase/decrease the speed of generator 2. One should observe a slow beat of the light brightness.
    3. When the lamps beat slowly, the switches should be closed when the lights are extinguished (line-line voltage at minimum).

     

         

     

    Paralleling switchgear

    synchronize and connect to a parallel bus via an ATS system.

    1. Utility breaker sensing

    No ATS, the system senses utility failure starts required generators, synchronizes them to paralleling bus and tie breaker closes to connect to load.

    1. Prime Power

    System starts all generator sets, the units synchronize and connect to paralleling bus.

    1. Isolate (Interruptible Rate) Starts, synchronizes and connects generators with the utility when a

    predetermined load level is reached and a utility breaker opens.

    1. Base Load (Peak Shave) Generator sets                start                and synchronize to bus, which then

    parallels generator bus with utility and generator bus tie breaker closes.

    The generator power remains constant, when less than the load the utility makes up the difference and when more the generators surplus power is fed into the utility.

     

    Reference: 

    • cpower.com
    • FG Wilson

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