Busbar replica for bus differential protection 87B

What the busbar replica is

In a single bus the zone of bus differential protection 87B is fixed: all the bay CTs bound the same bus. In a double bus, a triple bus or a double bus with transfer, each bay can be on any of the buses depending on the position of its bus disconnects. The protection needs a live image of the substation to know which currents to sum in each zone.

That image is the busbar replica (also called dynamic bus replica, or disconnect replica). The protection reads the position of each bus disconnect and each coupler, and with it assigns each bay CT to the corresponding zone. When a bay changes bus, its current moves from one zone to another.

Where the position comes from

• Hard-wired auxiliary contacts of the disconnect operating mechanism, one normally open and one normally closed, taken to the bay unit or to the central relay.
• GOOSE messages with the double-point position of logical node XSWI, in IEC 61850 substations. The position has four states: open, closed, intermediate and invalid.
• Mechanical replicas or mimics in older schemes, where a latching auxiliary relay copied the position.

In distributed schemes, each bay unit reads its CT and its disconnects and sends everything to a central unit; in centralized schemes, all the wiring goes to a single device. In both, the replica is only as reliable as the contacts and messages that feed it.

Early and late contacts

The tricky part is the interval in which the disconnect is moving. Current can start to flow before the blades touch, through the pre-strike arc, and it can keep flowing for a moment after they start to separate. The general rule of bus protection is:

• The disconnect is considered closed as soon as it is no longer fully open, that is, before its main contacts reach arcing distance.
• The disconnect is considered open only when the contact that confirms it shows it has gone past that distance.

That is why one speaks of an «early» contact for closing and a «late» one for opening. Bus protection manuals state which contact to use and at what point of the travel it must change over; the mechanical adjustment of the auxiliary contacts of the motor operating mechanism is part of commissioning.

With this rule, during a bay transfer between buses there is an interval in which the bay appears on both buses at once. The protection treats it as a single bus: it combines the two zones while the parallel lasts, and a fault on either one takes out both. That is the correct behaviour, because at that moment the two buses are joined through the bay.

Intermediate position, invalid position and discrepancy

A discrepancy appears when both contacts show the same thing, when neither changes or when the intermediate state lasts longer than the normal travel of the disconnect. Depending on the design, the protection only alarms, blocks the affected zone or blocks the whole bus protection. The philosophy is defined by the owner and must be written down.

The check zone as backup to the replica

The check zone sums all the bays in the substation without looking at the disconnects. It cannot tell which bus failed, but it confirms that the fault is inside the substation. When each zone trip also requires the check zone to operate, a replica error produces an alarm or a block, not a trip. It is the main defence against position errors.

What the control logic contributes

The replica is not only a protection matter. Substation control affects it in several ways:

• Bus transfer interlocks: the disconnect towards the new bus only closes with the coupler closed, so the bay never joins two buses at different potentials. The replica then goes through states the protection knows how to handle.
• Only one disconnect moving at a time: limits how long intermediate states last.
• Operating time supervision: a disconnect that takes longer than normal raises an alarm in control and in protection.
• Consistency between the position used by control and the one used by the 87B: if they come from different contacts, they can disagree. Ideally both read the same information or the difference is supervised.

What goes wrong in practice

Replica failures are the most frequent cause of 87B trips during switching. Misadjusted auxiliary contacts, reversed wiring, an intermediate position treated as open or a GOOSE message with invalid quality are enough to assign a current to the wrong zone. The diagnosis of those cases is in 87B trips when operating disconnects.

How it is tested

In general terms, the test runs each disconnect through each state, checks that the protection assigns the bay to the correct zone, measures at what point of the travel each auxiliary contact changes over, forces discrepancies and intermediate positions to check alarms and blocks, and uses secondary injection to confirm that the check zone and the selective zones act as expected with the bay on one bus, on the other and on both.

We can help with your bus protection

At Tiza Group we do the remote relay configuration of the replica, the zones and the discrepancy logic on the main bus protection brands, and the testing and commissioning of the 87B with the complete replica, in high and extra-high voltage and in industry, in Colombia or elsewhere on request. If your 87B tripped during switching, we can quote a relay trip analysis.

Related guides

• Bus differential protection 87B
• Busbar protection schemes: high and low impedance
• Double bus interlocking: coupler and transfer
• Bay and station interlocking: differences
• Sectionalized double bus and triple bus interlocking
• Substation interlocking: types, levels and logic

References

• IEEE C37.234, Guide for Protective Relay Applications to Power System Buses.
• IEC 61850-7-4, Compatible logical node classes and data object classes (PDIF, XSWI).
• IEC 61850-7-3, Common data classes (double-point position).
• L. Steenkamp, C. Labuschagne and E. Stokes-Waller, Tutorial: Complex Busbar Protection Application, Western Protective Relay Conference, 2007.
• C. Martin et al., Bus Protection Considerations for Various Bus Types, Western Protective Relay Conference, 2013.

Frequently asked questions