We claim:
1. An energy storage system (100), comprising:
a first energy storage system (110) comprising at least one of a battery and an engine generator;
a range extender (120) between the first energy storage system (110) and an electric load (160), comprising:
a first DC-to-DC converter (212) selectably coupled between the first energy storage system (110) and the electric load (160); a second energy storage system (220) comprising an ultra-capacitor; a second DC-to-DC converter (222) selectably coupled between the second energy storage system (220) and at least one of an output side of the first DC-to-DC converter (212) and an input side of the electric load (160); and
a by-pass (230) selectably coupled between the first energy storage system (110) and at least one of the output side of the first DC-to-DC converter (212), an output side of the second DC-to-DC converter (222), and the input side of the electric load (160); a controller (150) configured to control the first energy storage system (110) and the range extender (120) to work in at least one of a normal operation mode and a fault protection mode, wherein
in the normal operation mode, the first and second energy storage systems (110, 220) are parallel coupled to the electric load (160) through the first and second DC-to-DC converters (212, 222), respectively, and in the fault protection mode, the first energy storage system (110) is coupled to the electric load (160) through the by-pass (230), and the second energy storage system (220) is decoupled from the electric load (160); a first switch device (214) between the first energy storage system (110) and the first DC-to-DC converter (212);
a second switch device (224) between the second energy storage system (220) and the second DC-to-DC converter (222); and a third switch device (234) in the by-pass,

characterized in that in the normal operation mode, the first and second switch devices (214, 224) are closed and the third switch device (234) is opened, and in the fault protection mode, the first and second switch devices (214, 224) are opened and the third switch device (234) is closed, wherein upon a fault is detected, opening the first and second switch devices (214,224) a period after the third switch device (234) is closed.
2. A method of operating energy storage system (100), as claimed in claim 1,
comprising:
operating an energy storage system (100) comprising a first energy storage system (110), a first DC-to-DC converter (212), a by-pass (230) parallel to the first DC-to-DC converter (212), a second energy storage system (220) and a second DC-to-DC converter (222) in a normal operation mode, comprising parallel powering an electric load (160) by the first and second energy storage systems (110, 220) through the first and second DC-to-DC converters respectively (212, 222); and
operating the energy storage system (100) in a fault protection mode when a fault is detected in any of the second energy storage system (220) and the first and second DC-to-DC converters (212, 222), comprising powering the electric load (160) by the first energy storage system (110) through the by-pass (230) and decoupling the second energy storage system (220) from the electric load (160).
3. The method as claimed in claim 2, wherein upon detecting a fault, decoupling the second energy storage system (220) from the electric load (160) a period after the electric load (160) is powered by the first energy storage system (110) through the by-pass (230).
4. The method as claimed in claim 2, further comprising:
removing the fault;
closing the first and second switch devices (214, 224); and
opening the third switch device (234) a period after the first and second
switch devices (214, 224) are closed.

5. The energy storage system (100) as claimed in claim 1 or the method according to claim 2, wherein in the normal operation mode, the first and second energy storage systems (110, 220) supply average power and dynamic power to the electric load (160), respectively.
6. The energy storage system (100) as claimed in claim 1 or the method according to claim 2, wherein the by-pass (230) links a first point between the first energy storage system (110) and the first DC-to-DC converter (212) and a second point between the first and second DC-to-DC converters (212, 222) and the electric load (160).
7. The energy storage system (100) as claimed in claim 1 or the method according to claim 2, wherein the by-pass (230) comprises the third switch device (234).