All the machine parameters are fine. When changing fuel from NG to HSD, AHOP starts. GCV SRV parameters are ok. Main Hydraulic pressure is also ok.
What may be the cause for AHOP start/hydraulic oil pressure low sense?
Well, I reckon something is causing the Aux. Hyd. to start during the transfer, so that is what you need to find out from the software. There is no point in saying this OK, that is OK, the aux is starting and something is starting it!
What happens when you change from gas fuel to High-Speed Diesel?
As glenmorangie says, something is causing the Aux. Hydraulic Oil Pump to start. For most digital Mark* turbine control systems that something would be a pressure switch, usually 63HQ-1. Have you had someone verify that the pressure switch is calibrated properly and working properly?
What have you done to try to troubleshoot the problem--and, What were the results of your troubleshooting?
Does the unit have a hydraulic accumulator? If so, is it properly charged and are the block- and bleed (isolation and drain) valves in the proper positions?
It could be that the Main Hydraulic Oil Pump's compensator is not working properly. That's difficult to verify, but it could be a cause. How old is the Main Hydraulic Oil Pump?
There is usually a relief valve and an air-bleed check valve in the discharge of each hydraulic oil pump (see the Hydraulic System P&ID). Are you certain the relief valve of the Main Hydraulic Oil Pump is not relieving? TOO MANY people mistakenly believe the hydraulic system pressure is to be set using the relief valve--the pump compensator is used to set the pump discharge pressure of each pump. The relief valves are to protect against a failure of the compsensator that results in excessive pump discharge pressure. If the relief valve is relieving during normal operation of the pump the flow through the pump is much higher than normal and any additional flow--like to the SRV/GCV--may cause the Aux. Hyd. Oil Pump to start.
Please write back to let us know what you find and how you resolve the problem. If you require additional help you need to provide the details of what you've done to try to resolve the problem AND the results of your troubleshooting--not just "the system is okay" because as glenmorangie says, it's not okay if the Aux. Hyd. Oil Pump is starting during a fuel changeover.
CSA's post reminded me of one thing on these older machines, when the Aux. Hyd. cuts in it "latches" and needs an operator to open the latch.
I'm sure this has just been a hydraulic "bump" as the liquid fuel cut in. If you haven't adjusted the Pressure regulator on the Main Hyd. pump for a while, I would give that a go first.
Is HSD "high speed diesel" as CSA stated, or is it "high sulfur diesel" or something else? I'm not sure it makes any difference, but if it is high sulfur fuel, there may be fuel treatment equipment that has to start up, though I doubt that it would use GT control oil.
You stated that main hydraulic pressure is OK, but where is the pressure being measured? If it is at the main pump discharge, that doesn't tell us what the pressure is at the location of the 63HQ-1 pressure switch that is usually the device that starts the aux hydraulic pump.
What type of control is on the gas turbine? And, is this the original control or an upgrade? Mark I and Mark II controls would have had the 63HQ-1 pressure switch wired directly to the motor control starter for the aux hydraulic pump, with a separate contact wired to the turbine control panel for alarm purposes.
If there is an accumulator in the system, is it charged? I don't know how much the 2nd stage nozzles would move during a fuel transfer, but if they do move they would likely put a bigger load on the hydraulic system than the liquid fuel bypass valve.
Is this a new problem? If so, how long has it been since you were able to transfer fuel without the aux hydraulic pump starting? What may be different?