Comparison between high impedance and low impedance bus differential protection

High impedance bus differential relays are used on most of the busses in North America. The popularity can be explained by the good performance of this scheme in relation to CT saturation and the low cost, if used on a simple bus system. However with the introduction of numerical relays and their low CT burden as well as their ability of measuring several feeder currents, a low impedance bus differential principal could be applied on simple busses also. In addition, the availability of fiber optic communication inside of the substation allows the use of decentralized bus system in which the CT output values becomes transmitted via communication to a centralized unit versus having CT wires running to the control house. Low impedance bus differential protection systems have many positive attributes. Common advantages of all low impedance bus protection schemes are the ability to be able to use CT's of different ratios on respective branch inputs and the fact that the same CT used for the bus protection can be shared with the feeder protection relay. The question needs to be discussed what selection criteria needs to get evaluated for the selection of the appropriate principal. The paper will give an overview on both principles and explain the difference in building and evaluating of the differential current. The effect of CT saturation during external and internal faults will be discussed in details for both schemes as well as the CT requirements. Some advanced numerical techniques employed in low impedance bus differential relays to recognize CT saturation on heavy through-faults and avoid false tripping are presented. Common guidelines for setting both principles will be reviewed and explained in detail and some examples will be shown. The influence of the complexity of the bus system on the selection will be discussed. Benefits and disadvantages of both systems will be compared. The comparison will also include the difference during the installation, commissioning, operation and maintenance phase of the different systems. The evaluation will be done technically as well as economically.