Test switches and bay isolation for relay testing

Why a bay must be isolated before it is tested

An in-service protective relay does not work alone. Besides tripping its own breaker, it initiates breaker failure 50BF, which in turn trips a whole bus; it sends transfer trips or permissive signals to the remote end of the line; it blocks or unblocks schemes in other bays; and it reports events to the control center. Injecting current into that relay without isolating it can open the bay, the entire bus or the line at the far end.

Isolation has two goals: the test must not cause real operations, and the relay must receive only the quantities from the test set, none from the power system.

What is isolated

• Trip and close outputs to the bay’s own breaker.
• 50BF initiates to the bus protection or the breaker failure logic, and the reception of 50BF trips from other bays.
• Transfer trips and teleprotection channels to the remote end.
• Signals to other bays: reverse interlocking bus protection blocks, 87B initiates, interlocking permissives.
• Current circuits: CT secondaries are shorted on the CT side before being isolated from the relay. An open CT secondary under load produces dangerous voltages.
• Voltage circuits: they are isolated on the VT side so that the injection does not back-feed the voltage transformer secondary.
• Signals to the control center: the operator is informed, or they are flagged as test, so that events are not taken as real.

Physical test switches

In a hardwired control and protection panel, the circuits of each relay pass through a test switch or test block. Two widespread design families are mentioned here only as examples:

• Test blocks with a test plug, such as the Hitachi Energy COMBITEST RTXP family. When the plug is inserted, the block opens the trip circuits, shorts the CTs before the voltages are opened and leaves the relay terminals available for injection, in that order and mechanically.
• Individual knife-blade switches, such as the ABB FT family. Each circuit has its own blade; the current blades have a shorting contact that closes before the circuit opens, so the CT is never left open. The technician follows the opening sequence.

In both cases, the switch also lets instruments be inserted to measure in-service current without opening the circuit.

Virtual isolation in IEC 61850

In an IEC 61850 substation, many signals between bays are not wires but GOOSE messages. A physical test switch does not cut them. The standard has its own test mechanisms for that:

• Test mode of the relay or of a function: the function is put in test and its outputs carry test quality. Subscribers that are not in test do not use them.
• Test with blocked outputs: the function operates and publishes, but does not act on its physical outputs.
• Simulation: the relay accepts messages simulated by the test set instead of the real ones, which allows testing without disconnecting the network.

The result depends on every device implementing these modes consistently and on subscribers being configured to handle test quality. The typical failures of that configuration are described in GOOSE not arriving or arriving late. In digital substations with a process bus, current and voltage isolation is also virtual, because they arrive as sampled values.

How it comes together in practice

The isolation plan is specific to each bay and each substation. It comes from the drawings and the communications configuration, and it must be kept current: a signal added in an expansion and left out of the plan is the one that trips the bus during the test.

Risks when restoring

Restoration is the most dangerous part, because a mistake leaves the protection in service but blind or mute:

• A CT left shorted: the relay does not see the current and does not trip for a real fault.
• A trip blade or plug left open: the relay operates, but the breaker does not open. It is one of the causes reviewed in the protection did not trip for a fault.
• A relay left in test mode: its messages are ignored.
• A blocked teleprotection channel: the line clears with zone 2 times.
• An isolated 50BF initiate: if the breaker fails, nothing backs it up.

That is why restoration is done against the isolation log, point by point, and ends with a check: in-service current measured through the test switch, the state of each trip, the relay mode and the alarms at the control center.

How it is documented

The test report should include the isolation plan, the restoration log and the final check, in addition to the injection results. What to ask for in that report is summarized in what to ask for in a protection test report.

We can help with your in-service testing

At Tiza Group we perform secondary injection and protective relay testing in energized substations, with an isolation plan, a restoration log and a final check, at medium, high and extra-high voltage and in industry, in Colombia, or elsewhere on request.

Related guides

• Breaker failure protection 50BF (RBRF): how it works
• IEC 61850 test mode: Mod, Beh, Sim and the test bit
• Interlocking tests: what is checked at FAT and SAT
• Substation interlocking: types, levels and logic
• Interlock bypass: release, risks and control

References

• IEC 61850-7-4, Compatible logical node classes and data object classes (logical node behavior and mode, simulation).
• IEC 61850-8-1, Specific communication service mapping (SCSM) – Mappings to MMS and to ISO/IEC 8802-3.
• IEC 60255-1, Measuring relays and protection equipment – Part 1: Common requirements.
• Hitachi Energy, COMBIFLEX test switches RTXP 8, 18 and 24, product documentation.
• ABB, Type FT-1 Flexitest switch, product documentation.

Frequently asked questions