One of our clients asked is it possible to start a pump motor by directly connected power cable to diesel generator set without any means (breaker)? The diesel generator set is 1250kW and the motor is 630kW.
Initially I thought what an idiot and then stopped to pause. What are the design requirements and are you retro-fitting the pump to an existing diesel generator. The process of soft starting the pump using the DG should be able to be done if you are working from first principles. Rule 1. Trust nobody. You need to check ALL of this.
Define the operation of your pump. Here, from the limited information, I am assuming a centrifugal pump. Valve closed to start. The liquid density will not change quickly and the non-return valve is functional. Now you need to establish the run up time. This of course is complicated by the voltage. For this turn to the standard soft starter manuals.
Now to the diesel generator. You need to decide what rating. Baseload, Prime or Standby. As you only require the power for starting and then you will drop down to 630kW. The Standby allows you to take your 1120kW to 1600kW. Using this mode you are limited to 500 hours per year and then you require a full service.
You will require some sort of a breaker to disconnect the motor from the diesel generator in the event of a motor failure. This should have some sort of protection device to detect the overload of the motor as you can, in theory go to 1120kW before the generator begins to feel it. If you have this breaker, a little money can motorize it to be used to start the pump. Everyone, including myself, would recommend this.
If you don't have this, beside prayer, you need to look at the settings of your AVR. If you set the regulation to a very slow ramp you should be able to get this to run as the engine is large enough. The chance of instability of the AVR, because of load, is limited as it is a singular load. Sudden load / flow changes can send the engine into instability because of these settings.
Using all of this, look at the capability curve of the alternator to see if you can operate within the stability area.
A soft starter would definitely assist and would have all the protection you require for the motor but if you have no money for contactors or protection, this will be too expensive for you.
You may have to look at your cables as there may be excessive current flow for an extended duration. Depending on the distance, going for larger cables may be more expensive than motorizing the breaker.
A modern diesel engine with turbos and an electrical governor it will ramp fast enough. If you have the backing from your boss I would really give it a go with all the information above taken into consideration and a final check on the engine safety under speed and stall protection.