CHAPTER 5: SETTINGS GROUPED ELEMENTSL90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 5-22155.7.3.2 Stub busSETTINGS GROUPED ELEMENTS SETTING GROUP 1(6) LINE DIFFERENTIAL ELEMENT STUB BUSThe stub bus element protects for faults between two breakers in a breaker-and-a-half or ring bus configuration when theline disconnect switch is open. At the same time, if the line is still energized through the remote terminal(s), differentialprotection is still required (the line can still need to be energized because there is a tapped load on a two-terminal line orbecause the line is a three-terminal line with two of the terminals still connected). Correct operation for this condition isachieved by the local relay sending zero current values to the remote end(s) so that a local bus fault does not result intripping the line. At the local end, the differential element is disabled and stub bus protection is provided by a user-selectedovercurrent element. If there is a line fault, the remote end(s) trips on differential but local differential function and the DTTsignal (if enabled) to the local end are blocked by the stub bus logic, allowing the local breakers to remain closed.STUB BUS FUNCTION — There are three requirements for stub bus operation: the element must be enabled, an indicationthat the line disconnect is open, and the STUB BUS TRIGGER setting is set. There are two methods to set the stub bus triggerand thus setting up stub bus operation, as follows:1. If STUB BUS TRIGGER is “On,” the STUB BUS OPERATE operand picks up as soon as the disconnect switch opens, causingzero currents to be transmitted to remote end(s) and DTT receipt from remote end(s) to be permanently blocked. Anovercurrent element, blocked by disconnect switch closed, provides protection for the local bus.2. An alternate method is to set STUB BUS TRIGGER to be the pickup of an assigned instantaneous overcurrent element.The instantaneous overcurrent element must operate quickly enough to pick up the STUB BUS OPERATE operand,disable the local differential, and send zero currents to the other terminal(s). If the bus minimum fault current is abovefive times the instantaneous overcurrent pickup, tests have confirmed that the STUB BUS OPERATE operand alwayspicks up correctly for a stub bus fault and prevents tripping of the remote terminal. If minimum stub bus fault currentis below this value, then use method 1. Note also that correct testing of stub bus operation, when this method is used,requires sudden injection of a fault currents above five times instantaneous overcurrent pickup. The assigned currentelement needs to be mapped to the appropriate output contact(s) to trip the stub bus breakers. It should be blockedunless disconnect is open. To prevent 87L tripping from remote L90 relays still protecting the line, assign the auxiliarycontact of line disconnect switch (logic “1” when line switch is open) to block the local 87L function by using theCURRENT DIFF BLOCK setting.STUB BUS DISCONNECT — Selects a FlexLogic operand to represent the open state of auxiliary contact of line disconnectswitch (logic “1” when line disconnect switch is open). If necessary, simple logic representing not only the line disconnectswitch but also the closed state of the breakers can be created with FlexLogic and assigned to this setting.STUB BUS TRIGGER — Selects a FlexLogic operand that causes the STUB BUS OPERATE operand to pick up if the line disconnectis open. It can be set either to “On” or to an instantaneous overcurrent element (as outlined). If the instantaneousovercurrent used for the stub bus protection is set with a time delay, then STUB BUS TRIGGER should use the associatedinstantaneous overcurrent pickup operand. The source assigned for the current of this element must cover the stubbetween CTs of the associated breakers and disconnect switch. STUB BUS STUB BUS FUNCTION:DisabledRange: Disabled, Enabled STUB BUS DISCONNECT:OffRange: FlexLogic operand STUB BUS TRIGGER:OffRange: FlexLogic operand STUB BUS TARGET:Self-resetRange: Self-reset, Latched, Disabled STUB BUS EVENTS:DisabledRange: Disabled, Enabled
