2 BUSBAR PROTECTIONA protection scheme for a power system should cover the whole system against all probable types of fault.Unrestricted forms of line protection, such as overcurrent and distance systems, meet this requirement, butclear faults in the busbar zone only after a time delay. If unit protection is applied to feeders and plant, thebusbars are not inherently protected.Busbars are often left without specific protection since the risk of a fault occurring on modern gear isconsidered to be small. However, the damage resulting from one uncleared fault may be extensive.Busbar protection is needed when the system protection does not cover the busbars or when high-speedfault clearance is needed to maintain power system stability. Unit busbar protection provides this. Also if thebusbars are sectionalised, only one section needs to be isolated to clear a fault.Busbar faults generate large fault currents and must be cleared quickly, otherwise the whole substation is atrisk due to both dynamic forces and thermal effects. Therefore busbar protection must provide high-speedoperation. A false trip on a distribution bus can affect many customers as feeders and transmission lines aredisconnected. Also a false trip on a transmission busbar can seriously affect power system stability.Therefore busbar protection must provide maximum security.2.1 UNIT PROTECTIONIn large power networks, tripping times of protection equipment can vary depending on the distance from thefault. To reduce this, power networks can be divided into protection zones. Values can be compared at zoneboundaries to narrow the fault to a specific zone. Each zone can detect and isolate its own faults. The zonecan cover several items of equipment such as a busbar or just one item such as a transmission line,transformer, motor or generator. This is known as Unit Protection.2.2 CURRENT DIFFERENTIAL PROTECTION PRINCIPLESOne form of Unit Protection is called Current Differential Protection. This was first defined by Merz and Priceusing Kirchoff's current law. It compares currents entering and leaving a unit. If there is no fault, the currentthat enters is the same as the current that leaves, so the difference is zero. If there is a fault, the difference isequal to the fault current. The following diagram shows two current transformers (CTs), one at each end of aprotected zone. If there is a fault between the two CTs, a current flows through the IED.V00712CB ACT A CB BSourceLoadIEDCT BTrip TripIL ILILIL/n IL/nIdiffProtectedequipmentFigure 27: Current Differential ProtectionThe differential scheme creates a well-defined protection zone between the two CTs. Any fault in thedifferential protection zone is an internal fault, while any fault outside is an external fault. The protectionneeds to operate only for faults in the protection zone and to be sensitive to low fault currents. Theoreticallya differential scheme should respond to the smallest internal faults but restrain on the largest external faults.Chapter 5 - Protection Functions MiCOM P747100 P747-TM-EN-1
