Hybrid magnetic-optical sensor
This type of transformer is mostly used in applications such as series capacitive compensation of long transmission lines, where a non-grounded of long transmission lines, where a non-grounded measurement of current is required. In this case, several current sensors are required on each phase in order to achieve capacitor surge protection and balance. The preferred solution is to use small toroidally wound magnetic core transformers connected to fibre optic isolating systems. These sensors are usually active sensors in the sense that the isolated systems require a power supply. This is illustrated in figure 6.26.
6.5.2.3. Rogowski coils
The Rogowski coil is based on the principle of an aircored current transformer with a very high load impedance. The secondary winding is wound on a toroid of insulation material. In most cases the Rogowski coil will be connected to an amplifier, in order to deliver sufficient power to the connected measuring or protection equipment and to match the input impedance of this equipment. The Rogowski coil requires integration of the magnetic field and therefore has a time and phase delay whilst the integration is completed. This can be corrected for within a digital protection relay. The schematic representation of the Rogowski coil sensor is shown in figure 6.27.