Interlocking in primary selective and MV open-loop systems

Interlocking in primary selective and MV open-loop systems covers the two usual ways of giving the transformers of a medium-voltage network a second supply. In a primary selective system each transformer can take one of two feeders. In a medium-voltage open loop the transformers hang from ring main units (RMUs) on a cable that leaves one bus and returns to another, with a normally open point. Both share one rule: never tie two feeders together unless it was planned.

Primary selective: interlocked selector switches

Each transformer reaches feeders A and B through two selector switches (89SA and 89SB), followed by its earthing switch, its primary breaker and the transformer itself. The interlocking logic, with selectors that have no load-breaking capability:

Selection is done dead: the transformer breaker is opened, the selector is changed and the breaker is closed again. The load sees a short interruption, that of an open transition transfer.

If the selectors are load-break switches or breakers, the design can allow the change with the transformer in service, but the «other one open» condition stays. A duplex selector, a single device with positions A, open and B, meets the condition by construction.

When the change is automatic, an undervoltage logic opens the selector of the lost feeder and closes the other one. That automatic transfer is blocked if the transformer protection saw an internal fault: moving the transformer to the other feeder would close it onto the fault.

Medium-voltage open loop: RMUs and the open point

Each RMU has two ring load-break switches, one towards each side, each with the earthing switch of its cable, and a transformer function with a switch-fuse combination or a circuit breaker. A medium-voltage load-break switch makes and breaks load current, loop current and cable charging current, but it does not interrupt a short circuit.

In many RMUs the switch and the earthing switch of a cable are a single three-position device (closed, open and earthed), so by construction it only goes from closed to earthed through open. IEC 62271-200 also requires interlocks between the switching devices of the switchgear and its earthing. What the switchgear cannot see is the far end of the cable, in another RMU or at the source substation: a cable has two ends, and it is earthed at both after verifying the absence of voltage.

The normally open point

Moving the open point is the most frequent switching operation on the loop, and the order matters:

If the new point is opened before the old one is closed, the RMUs between the two lose supply.

Never close the loop between different sources without a study

With the bus section breaker of the main substation closed, both ends of the loop come from the same bus and closing the open point is a loop closure. With the bus section open, or if the two feeders come from different substations, closing the open point parallels two sources with possible voltage and angle differences. The load-break switch has no synchronism check, and the circulating current can be high. That closure is prevented by interlocking or by procedure, unless a study allows it.

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

Bay level and station level

In both arrangements, the switchgear handles its own logic with mechanical interlocks. What it cannot handle is the network: a transformer selector does not know whether the other feeder comes from the same source, and the RMU does not know whether the far end of the cable is open. Those conditions live in the control centre, in the distribution automation or in the operating procedure.

Common mistakes

• Selectors without mutual interlocking, with a duplex replaced by two separate disconnects.
• Transfer without blocking on transformer fault.
• Cable earthed at one end only.
• Open point moved the wrong way, opening before closing.
• Loop closed between different substations without a study.

How it is tested

The mechanical interlocks of each switchgear unit are tested by attempting every prohibited operation, and the selection and transfer logic is tested with the loss of each feeder and with a transformer fault. In motorized RMUs, the remote controls and their indications are tested too.

We can help with your network

We load and change the logic, interlocks and signals remotely through our remote relay and controller configuration service, without waiting for a site visit. At Tiza Group we prepare the secondary engineering of medium-voltage networks (interlocking logic, transfers and SCADA integration) for industry and distribution utilities, adapted to the local code and the utility’s requirements. Commissioning is done in Colombia, or elsewhere on request. Contact us to discuss your project.

Related guides

• Medium-voltage switchgear and RMU interlocking
• FLISR: fault location, isolation and service restoration
• Automatic transfer in medium voltage: logic and blocking
• Main-tie-main interlocking: secondary selective
• Interlocking tests: what is checked at FAT and SAT

References

• IEC 62271-200: AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV (switchgear interlocks).
• IEC 62271-103: switches for rated voltages above 1 kV up to and including 52 kV.
• IEC 62271-105: alternating current switch-fuse combinations.
• IEC 60050-605: substation vocabulary (ring).
• IEEE Std 141 (Red Book): electric power distribution for industrial plants (primary selective).
• EN 50110-1: operation of electrical installations (five safety rules).

Frequently asked questions