International standards for short-circuit analysis in ac power systems

Power station units with full size converters such as type 4 wind turbines and solar PV converter-transformer station units are modelled in the positive-sequence system as a current source. The current is provided by the manufacturer. The positive-sequence shunt impedance is assumed infinite. In the case of unbalanced faults, the negative-sequence impedance depends on the design and control strategies and is given by the manufacturer. The zero-sequence impedance is infinite.

 

Solar power system units with full-size converters may be neglected if their contributions are smaller than 5% of the initial short-circuit without these units.

 

The condensers mentioned previously are direct-contact condensers. Shell and tube condensers can also be used to better control NCG emissions. However, shell and tube condensers are expensive and less efficient.

 

When a geothermal resource cannot produce a sufficient quantity of clean enough steam to drive steam turbines, other modes of power generations need to be used. If the steam contains too much NCG (>10% weight of steam), a condensing turbine unit becomes uneconomical. In such a case, a back pressure turbine that does not have an after-condenser can be used. The steam is exhausted directly to the atmosphere, but this mode of generation is unlikely to be acceptable nowadays for environmental reasons. Either total flow or binary mode of generation needs to be used.

 

Many geothermal ORC binary plants are in operation around the world. A heat exchanger (shell and tube type) is used to transfer heat from the geothermal fluids to a low-boiling point secondary fluid (isopentane or isobutane). The secondary fluid superheated vapor drives a vapor turbine that exhausts to an air-cooled shell and tube condenser. The secondary fluid is in a closed loop, and the spent geothermal fluids are reinjected.

 

Whereas an ORC binary plant usually uses an air-cooled condenser, a typical condensing steam turbine unit uses a direct-contact condenser. The resultant condensate/cooling water mixture is normally removed from the condenser via a barometric leg or a pump. This condensate mixture flows to a holding pond known as the hot well, and from there it is pumped to a cooling tower to be cooled for recirculation to the condensers. As the geothermal steam from the steamfield eventually becomes a part of the recirculating cooling water, the total volume of the recirculating water tends to increase, even though some is evaporated at the cooling tower. Hence, some of the condensate mixture is reinjected from the hot well. Because this condensate mixture is warm (∼40°C) and oxygenated, it is quite corrosive to carbon steel. Stainless steel pumps, pipes, and well casing are required for condensate reinjection.

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