H Configuration Interlocking: Bridge and Two Sources

H configuration interlocking focuses on the crossbar of the H. In the H configuration two lines each arrive at a node, a transformer leaves from each node and a bridge ties the two nodes. The bridge decides whether the substation works as two independent supplies or as one with both lines in parallel, and the control logic decides when it can change from one mode to the other.

Variants and what changes in the logic

The H has variants depending on where the circuit breakers are, and each one changes the interlocking:

• Full five-breaker H: each line, each transformer and the bridge with its own breaker. This is the variant described below.
• Three-breaker H: no transformer breakers or no line breakers. The disconnector on the side without a breaker operates only with the bridge and that branch’s breaker open.
• Disconnector bridge: the bridge has no breaker and is operated only without current.

Conditions per device (full H)

Each node is a small bus between three breakers. Its earthing switch is the only condition that mixes three bays.

The full table for this arrangement, with the CILO equations and the list of test cases, can be generated with our free interlocking table tool.

The bridge as a parallel of sources

With the bridge closed and both lines in service, the two lines are in parallel through the substation. If the substation sits in the middle of a corridor, power can flow from one line to the other. Closing the bridge, then, is not just another operation:

The synchronism check of the bridge needs the voltage of each node. If the voltage transformers are on the line side, the voltage of each node is taken from the line of its branch through a voltage selection based on the position of its breaker and disconnectors.

Transfer between transformers

If the bridge works open, each line feeds its transformer. When one line is lost, the bridge can close so that the other line picks up both transformers. That automatic transfer lives in the bay controller or in a PLC and has its own permissives: no voltage on the affected node, the breaker of the lost line open, no fault on the node or the transformer, and the other line able to carry the load. See automatic source transfer.

With the bridge closed, the problem is the opposite: taking out a line interrupts nothing, but taking it out with the bridge open leaves its transformer without a source. Good logic warns before executing that command.

Bay level and station level

The interlocks of each line, each transformer and the bridge are bay level. Station level are:

• the node earthing switches, which need three bays;
• the bridge close permission, which needs the voltage and fault state of both branches;
• the automatic transfer.

With five bays, hardwiring is manageable. With IEC 61850 controllers, the bridge subscribes by GOOSE to the positions and protection pickups of both branches.

Critical switching sequences

Medium and high voltage

At high voltage, the H is typical of subtransmission, with motor-operated disconnectors and logic in the bay controllers. At medium voltage it appears in industrial plants with two utility feeds and two transformers, often in switchgear: there, each panel’s interlocks come from the manufacturer according to IEC 62271-200, and anything that depends on other panels, such as the bridge or the node earthing switch, is solved with electrical logic or trapped keys. In both cases, the interlocking cannot see the earth at the remote end of a line: that condition is confirmed with the control centre.

Typical mistakes

• Disconnector bridge without the isolated-node condition. The bridge is closed with both branches live.
• Transfer without fault blocking. The bridge closes onto a faulted transformer or node.
• Incomplete node earthing condition. The condition leaves out the bridge disconnector.
• Bridge 25 with a single voltage. Without the voltage of the other node there is no sync check and no dead-node detection.

How it is tested

The matrix of each device is covered, the bridge close is tested in the live and dead node combinations, and the transfer is tested with the loss of each line, with a fault present and with the bridge unavailable. Secondary injection covers the 25 and the blocking.

We can help with your interlocking logic

At Tiza Group we design H substation logic as part of secondary engineering: interlocking, bridge logic, transfer and sync check, and we program it through remote configuration, for subtransmission substations and industrial plants with two utility feeds. On-site testing is done in Colombia, or elsewhere on request.

Related guides

• Breaker failure protection 50BF
• Bay-level and station-level interlocking
• Tee connection interlocking
• Sectionalized busbar interlocking
• Interlocking tests: what is checked at FAT and SAT

References

• IEC 61850-7-4: logical nodes CILO, CSWI and RSYN.
• IEC 60050-605: substation vocabulary (four-breaker arrangements and their bridge variant).
• IEC 62271-102:2018: alternating current disconnectors and earthing switches.
• IEC 61936-1:2021: power installations exceeding 1 kV AC.
• EN 50110-1: operation of electrical installations (five safety rules).
• IEEE C37.2: device functions 25, 52, 57 and 89.

Frequently asked questions