Digital optics for control and protection systems
Measurement is a focal point in Alstom Grid’s 800 kV HVDC project. One of the most challenging tasks in developing HVDC systems is optimising the performance and size of the system’s transducers for control and measurement. The technology behind the transducers is optical sensors: The current transducer looks like a set of rings hanging from the voltage busbar. Alternatively it can be installed as a self-supporting device between the busbar and the ground. It responds at temperatures between 60°C and minus 40°C with a scale factor error of less than 0,1%, and boasts a frequency bandwidth from dc to 5 kHz. Other technologies, though powerful, had inherent limitations. Voltage insulation restricts the use of zero flux sensors to 500 kV, while resistance shunt technology does not measure harmonics accurately enough. The optical current transducer has emerged as the right solution: Simple, rugged, and very accurate.
- Optical current sensing is based on the degree of light polarisation introduced by the magnetic field.
- Optical current sensors have no galvanic connection to the conductor in which the current is being sensed.
- A 0,1% accuracy of measured current has been achieved.
Digital optics put control and protection systems in the picture
By P-A Monfils, Alstom
Electricity+Control, December 2012 (pages 29 – 30)