Hydrostatic drive system

1. A hydrostatic drive system, comprising:

a user comprising a differential cylinder connected in a closed circuit to a high-pressure pump, in which a piston-side compression chamber of the differential cylinder is in communication with the high-pressure pump by a first power fluid line, and a piston-rod-side compression chamber of the differential cylinder is in communication with the high-pressure pump by a second power fluid line,

wherein the closed circuit comprises a protection device comprising a first protection valve device that comprises a pressure relief valve and an anti-cavitation valve which is associated with the first power fluid line, and a second protection valve device which comprises a pressure relief valve and an anti-cavitation valve which is associated with the second power fluid line,

wherein the protection valve devices are connected with a low-pressure system, and

wherein the pressure relief valve of the first protection valve device associated with the first power fluid line includes an override device by means of which an opening pressure of the pressure relief valve is varied between a protection pressure of the closed circuit and a reduced opening pressure, whereby when the high-pressure pump delivers into the piston-rod-side compression chamber, the pressure relief valve of the first protection valve device is adjusted by the override device to the reduced opening pressure due to an actuation of a control element controlling an actuation of the user, and a differential volumetric flow flows out via the pressure relief valve from the first power fluid line into the low-pressure system, and the pressure relief valves of the first protection valve device and the second protection valve device have a protecting pressure that protects a high pressure of the closed circuit,

wherein the low-pressure system has a low-pressure pump that is operated in the open circuit and delivers into a delivery line, and a pressure protection device is associated with the delivery line,

wherein the pressure protection device of the low-pressure system is a pressure relief valve, the protection pressure of which is varied by an adjustment device,

wherein the override device of the pressure relief valve and the adjustment device of the pressure protection valve are actuated as a function of the actuation of the control element.

2. The hydrostatic drive system as claimed in claim 1 , wherein when the high-pressure pump delivers into the piston-side compression chamber, the differential volumetric flow in the second power fluid line is supplemented from the low-pressure system via the anti-cavitation valve of the protection valve device which is associated with the second power fluid line.

3. The hydrostatic drive system as claimed in claim 1 , wherein each protection valve device comprises a combined pressure relief and anti-cavitation valve.

4. The hydrostatic drive system as claimed in claim 1 , wherein the pressure relief valve of the first protection valve device is adjusted by the override device to an opening pressure which is below the protection pressure of the pressure protection device.

5. The hydrostatic drive system as claimed in claim 1 , wherein the override device is a hydraulic override device or an electrical override device.

6. The hydrostatic drive system as claimed in claim 1 , wherein the adjustment device is a hydraulic adjustment device or an electrical adjustment device.

7. The hydrostatic drive system as claimed in claim 1 , wherein the low-pressure pump is a variable displacement pump, the delivery of which is adjustable, and the delivery of which is reduced in the event of an excess volumetric flow to be removed from the closed circuit.