4. X-MODE VIBRATION CONTROL BY TOP BRAClNG AND SYNCHROPHASER
In this chapter, X-mode vibration control method is introduced by using synchrophaser and top bracing for 60 MW diesel power plant paralleled with two 9K80MC-S units whose arrangement is shown as Fig. 11.
X-mode resonance was situated around generator synchronized speed and so foundation was designed in initial stage having enough stiffness to separate these frequencies. However, strong structural 38m vibration were occurred in site and furthermore transferred to the adjacent buildings which had a bad effect on safety of them and working condition. It was found out by vibration measurement that these vibrations were mainly dependent on phase angle of X-mode vibration between two units and it was finally decided to control this by applying top bracing and synchrophaser.
Engine speed was to be mediated for this method and block diagram for this is shown as Fig. 12. Detail procedure to control phase angle was at first to take an electric load on referenced engine when phase and speed were not changed because it was connected to the grid of high rank. Phase angles and speeds of two engines were detected by synchrophaser and then by governor phase of unloaded engine was controlled same as the referenced engine. It was for phase difference between two engine to be zero in advance of finding out optimum value. And then took electric load on the other engine. The phase control could be done if phase difference was within 6.2 ｡?hich might be changed due to number of electric poles.
However, there were speed fluctuation and error of sensor by uneven cylinder load and torsional vibration and so phase difference was preferred to be within 3.0°(4.8 msec). Though phase difference between two engine was controlled within ±1 pole, it could be electric 2 poles if not exactly controlled. In this case, phase difference between two engine was 12.4°and 6th X-mode phase became 74.5° which showed quite different vibration aspect between engine and adjacent structures.
All the above achieved, top bracing was connected with a view of counterbalancing X-moment horizontally applied. Top bracing should be designed and made to have enough strength not to raise any distortion. With phase sensor fixed at No. 1 engine, phase angles of various point at No. 2 engine were checked by the other phase sensor. During this procedure, optimum phase angle was decided and fixed in a view of X-mode vibration for engine and adjacent buildings.
Vibration readings of 6th order after fitting top bracing and synchrophaser with adjusting phase difference between two engines are shown Fig. 13〜16 by polar coordinate. In these figures, #1 and #2 denote measured 6th phase difference with No.1 and No.2 engine isolated running and the others are measured 6th phase angle and amplitude when both two engine is running simultaneously.
Fig. 11 Layout of diesel power plant and adjacent buildings
Fig. 12 Block diagram for phase adjustment of X-mode vibration
