Steam Turbines and Harmonic Distortion

Dear Sirs,

We have 11KV 8MW x3 sets steam turbine alternator in service on one of our FPSO. this plant is being commissioned and one of the observation in relation with the subject as follows and I need your view.

2 Turbine were running on load around 5 MW total (2.5+2.5) and we observed some vibration on the turbine side and also vibration level indicators went high and reached alarm level, it continued few hours. Every one running around the plant with the worries and trying all the stunt on steam side, condensate side, exhaust side to make sure there is no water carry over in the system, lubrication side also checked and all found normal, nothing could make out. finally it is found out that we have down hole heater connected to 11KV system load was increased to 50% from 25% causing load raise approximately by .8MW and the vibration also started at this time onwards. in order to conform we have brought down the heater back to 25% operation and vibration stopped. we feel harmonic distortion on power controller causing this turbine to vibrate!!!!!!!!!!! in order to do counter check load on TA increased similar load as heater by using some pumps, but turbine remain calm. if we start heater again, we are getting vibration. what could be the reason??, is it harmonics??? can harmonics disturbance on electrical system can reflect on prime mover???? or some grounding problem with the heater can cause this trouble??? Turbine alternator remain calm and no sign of any vibration etc..other than turbine vibration Please write your views.

thanks and best regards

K.Parimalam