HVDC > HVDC Classic > HVDC technology > DOIT
ABB has developed simple, reliable and very accurate optical current and voltage transducers. They are a integrated part of the MACH 2 control and protection system.
ABB's DOCT (digital optical current transformer) consists of a transducer in the primary circuit, connected by an optical fiber to the interface unit in the control room. In the transducer, the current is measured with a magnetic current transformer, a shunt or a Rogowski coil. After sampling and conversion into digital form by the DOIT electronics, the current value is transmitted as an optical signal in the fiber to the interface in the control room. Power to supply the DOIT electronics is simultaneously transmitted as laser light from the interface to the transducer, using the same or a separate optical fiber. 
Block diagram, DOCT
In the DOVT, Digital Optical Voltage Transformer a capacitive or resistive voltage divider is used to measure the voltage. The values are sampled and transmitted to the interface unit in the control room using identical opto-electronics as in the DOCT. The transfer function is better than with conventional voltage transformers and the risk of ferroresonance is eliminated.
Block diagram, DOVT