A field case with fictional names: during secondary injection testing of a feeder, the busbar differential protection trips and every breaker in the zone opens. Current injected through a single measuring circuit looks like a bus fault if that circuit is still connected to the 87B. The ABB REB670 manual describes the means to avoid it: test switch, test mode and selective unblocking of functions.
The case
Mateo, a test engineer, works on a double-busbar 115 kV substation during commissioning. He has to test the relay of a new feeder with secondary injection. The busbar protection (87B) is already installed and partly in service; other feeders are energized. To go further: SIPROTEC 7SS85 and 7SS52 relays.
Mateo injects current into the feeder circuit. As the current rises, the busbar 87B trips and opens every breaker on that bus. There are load interruptions and a report to write.
First, do not repeat the test
Before trying again, review what happened. The 87B oscillography answers most questions:
- Which circuit showed current? If only one circuit in the zone shows current and the others are at zero, the 87B sees it as a bus fault, even though there is no fault.
- Was it the circuit of the feeder under test? If the circuit with current is the test one, the injection reached the 87B.
- Which zone tripped? The zone that contains the feeder according to the disconnector replica.
- Who was in test mode? The feeder relay, not necessarily the 87B.
See 87B trip when switching disconnectors for a neighboring case.
Why it happens
Busbar differential protection adds the currents entering and leaving the zone. In normal operation the sum is zero. The REB670 manual illustrates it in its tests: when current is injected into a single input, the differential function operates when the current rises to its threshold. That is right for the protection and bad for whoever is testing another module: an injection into a circuit still included in the zone looks like a fault.
The feeder circuit is connected to the zone because the disconnector replica says so: the 87B uses the auxiliary contacts of each busbar disconnector to know which currents to include in each differential zone (see disconnector replica for the 87B). If the disconnector of the feeder under test was closed, its current circuit still counted.
Possible causes
| Cause | What gives it away |
|---|---|
| Feeder measuring circuit not isolated from the 87B | Only that circuit shows current in the record |
| Busbar disconnector closed or replica set to “include” | The zone that tripped is the feeder’s |
| 87B trip not blocked during the test | The trip reached all breakers in the zone |
| Test made on the wrong relay | The feeder record shows no injection |
| Test blocks operated wrongly | Wrong order of opening and short-circuiting |
What the manufacturer says about testing safely
The ABB REB670 installation and commissioning manual includes means to avoid this situation:
- COMBITEST test device. When the handle is inserted, preparation is done in the proper sequence: trip circuits are blocked, current transformers are short-circuited, voltage circuits are opened and the terminals are enabled for injection. When it is removed, the safety latches leave it half-way out so any transient feedback decays before the trip circuits are restored.
- Without a test device, measures must be taken according to the circuit diagrams.
- Never open a current transformer secondary without short-circuiting it.
- Test mode. It blocks all functions and lets you unblock only those to be tested. The yellow LED flashes while it is active. Before each session, check that only the function under test and the interconnected ones are unblocked.
- Disconnector replica: test by manually operating the disconnectors or breakers and verifying that the switching status block feeds the corresponding binary inputs.
These measures belong to that maker’s device. On other makes the principle is the same and the means change.
How to keep it from happening again
- Is the feeder measuring circuit isolated from the 87B?
- No: Isolate it with the test device and record the operation
- Yes: Is the 87B trip toward the breakers blocked?
- No: Block the zone trip before injecting
- Yes: Inject and confirm in the 87B that it sees no current
Practical rules:
- A work order that names the 87B, even if the test is on another module.
- An isolation and restoration switching sheet, signed by whoever issues it.
- A prior test of the disconnector replica to know which zone includes which circuit.
- Confirm with the 87B readout that it sees no current before raising the injection.
- Restore and verify: after the test, restore the current and trip circuits in the order of the device, and check real currents under load. See busbar differential protection 87B.
What Mateo did
With the substation manager’s support, he restored the bus and documented the sequence with the 87B and feeder records. He then redid the test with the circuit isolated, the 87B trip blocked and a readout of the 87B before injecting. In the report he separated the cause (circuit not isolated) from the consequence (correct 87B trip).
When to call us
If your 87B tripped during a test or you need an isolation procedure for your yard, our team does secondary injection and trip analysis.
Related guides
- Busbar differential protection 87B
- 87B trip when switching disconnectors
- Disconnector replica for the 87B
- CTs for busbar and line differential protection
- Primary and secondary injection
- Commissioning of protection relays
- SIPROTEC 7SS85 and 7SS52 Busbar Protection Relays
References
- ABB, REB670 Installation and Commissioning Manual (1MRK505180-UES), sections on verification by secondary injection and primary injection testing.
- IEEE C37.234, Guide for Protective Relay Applications to Power System Buses.
Frequently asked questions
Why can a test on a feeder trip the 87B?
Because the 87B compares the currents of all circuits in the zone. If current is injected into the measuring circuit of a feeder that is still connected to the busbar protection and that current has no counterpart in another circuit, the protection reads it as differential current.
How is a relay isolated for testing according to the manufacturer?
The REB670 manual describes the COMBITEST system: inserting the test handle blocks the trip circuits, short-circuits the current transformers, opens the voltage circuits and enables the terminals for injection. Without a test device, measures are taken according to the circuit diagrams.
What does the disconnector replica show in an 87B test?
It shows which differential zone each circuit belongs to. The manual says to test it during commissioning by operating the disconnectors and checking that the switching status block feeds the corresponding binary inputs.