Trip oil & Hydraulic oil in GE frame 9 Gas tubines

Why we need to have a trip "control" oil in frame 9 gas turbines? why we can't just control the hydrauilic oil directely by solenoid valve to block it in trip cases?
Moreover, What is the use of trip oil pressure switches 63HG-1/2/3 protection?
If we lose the trip oil, the dump / stop valve will be closed anyway and trip the unit without this "trip oil low pressure" protection.

Can anyone explain please?

The simplest answer is that units with mechanical overspeed bolts use Trip Oil (sometimes called 'Control Oil') as the means of immediately shutting off the flow of fuel to the unit in the event of an overspeed condition not detected by the turbine control system. Using Trip Oil to trip the turbine in the event of an overspeed condition detected by the mechanical overspeed bolt makes it independent of the turbine control system. So, if the turbine control system was not working (say the 125 VDC breaker supplying the Mark* IV had tripped and for some really odd reason the fuel stop valve didn't close because the electrical signal to it wasn't working) and the unit went into overspeed the mechanical overspeed bolt would relieve Trip Oil pressure, which would allow the dump valve to remove hydraulic pressure from the fuel stop valve actuator and the fuel stop valve's spring would slam it closed, thereby shutting off the flow of fuel to the turbine.
I neglected to say I won't try to defend the usage of three redundant Trip Oil pressure switches in the fuel shut-off valve circuits; it always seemed like overkill to some of us. To be honest, I think the designers of the Mark IV went a little overboard with the TMR thing on that system.

The reason it seems like so much overkill is that during firing the Mark* IV doesn't consider Trip Oil system pressure--it allows the fuel to flow even if Trip Oil pressure is low! BUT, if the Trip Oil pressure is still low (on any of the three switches...) AFTER firing is complete the unit will be tripped--on low Trip Oil pressure. I "learned" that very early in my career, and it was a very painful lesson that I've never forgotten. (I think the reason Trip Oil pressure is "allowed" to be low during firing is that those units with black start capability can't always establish Trip Oil pressure above 20 psig as firing is initiated (I think that's the actuation setpoint) during STARTing and firing, but as speed increases during firing Trip Oil pressure will increase (because it's supplied by the output of the Main L.O. Pump, which for many units is Accessory Gear-driven). But, it's still kind of a jumbled bunch of logic and redundant pressure switches--another one of those system that has been around for decades and decades and people are loathe to change it because it's been around for so long and nobody who was around when it was designed is left to tell the story of how it came to be how it is. So, "If it ain't broke, don't fix it," right? (Some people's definition of "broke" is, ..., well, ..., broken!)