Synchrocheck voltage selection in double bus

Short answer

The 25 function must compare the voltages of the two points the breaker is about to tie. In a double bus, the bus side is the bus the bay is connected to at that moment, and the selection follows the position of the bus disconnectors. In breaker-and-a-half and ring bus, each breaker can have a different bus or line on each side depending on the topology. If the selection is wrong, the relay compares the wrong voltage and may allow an out-of-step close.

Why the voltage has to be selected

The synchronism check 25 compares the voltage on both sides of an open breaker. In a single bus this is direct: one voltage transformer on the bus and another on the line. But in arrangements with more than one bus, or where a breaker does not have a fixed bus on each side, the point the breaker is about to tie changes with the topology. The 25 function must measure at that point and not at another.

If it compares the wrong voltage, two things can happen. The less serious is that it never allows closing, because it sees an angle or a magnitude that does not match. The serious one is that it allows closing when it should not, because the voltage it measures is in synchronism with the line but the real bus is not. That is an out-of-step close, with all its consequences for transformers and generators.

Double bus

In a double bus there is a voltage transformer on each bus. Each line bay has its own on the line side. The bay’s 25 compares the line voltage with that of the bus the bay is connected to, and that bus is known from the position of the bus disconnectors 89B1 and 89B2.

Bus voltage selection for the 25 in a double bus

Which bus disconnector of the bay is closed?
- Only the bus 1 oneBus 1 reference
- Only the bus 2 oneBus 2 reference
- Both, during a transferReference from the bus with priority by design, without paralleling the secondaries
- Neither, or invalid positionNo valid reference: the 25 blocks closing or raises an alarm

The National Grid synchronizing specification, for example, requires the reference voltage to be selected from the position of disconnectors and breakers, using their auxiliary contacts, and a simple priority to be applied when there are no bus voltage transformers, so that the secondaries of the circuit voltage transformers are not paralleled.

The bus coupler deserves its own attention: its 25 compares bus 1 with bus 2 and does not depend on a selection, except in sectionalized bus arrangements, where each section may or may not have its own voltage transformer.

Breaker-and-a-half

In the breaker-and-a-half arrangement, each diameter has three breakers between two buses and two circuits. None of them has a fixed side:

Bus 1 side breaker. It ties bus 1 to circuit A. It compares the bus 1 voltage with that of circuit A. If the circuit A disconnector is open and the center breaker is closed, the other side is no longer circuit A but whatever lies behind the center breaker: circuit B or bus 2.
Center breaker. It ties circuit A to circuit B. If one of the circuits is isolated, the reference moves to the bus on that side through the other breaker.
Bus 2 side breaker. Symmetrical to the first.

The selection then depends on the position of the line disconnectors and of the other two breakers of the diameter. It is one of the logics most prone to design errors, because the combinations grow quickly and some only appear during maintenance. The same topology drives the breaker-and-a-half interlocking.

Ring bus

In a ring bus, each breaker sits between two nodes and each node generally has a circuit with its own voltage transformer. If a node’s circuit is isolated, that node loses its own reference and the selection must look for the next available measurement around the ring, following the closed breakers. Some designs use voltage transformers at every node to simplify the logic.

Hardwired or by sampled values

Hardwired selection. Auxiliary relays switch the voltage transformer secondaries to the relay input, driven by the disconnector contacts. The risks are paralleled secondaries at the instant of switching, misadjusted auxiliary contacts and a switchover left halfway. That is why schemes use an explicit priority and supervise the selected voltage input.

Logic selection. In modern relays and controllers, all voltages enter the device, or arrive over the network, and the selection is internal logic. In a digital substation with IEC 61850-9-2 sampled values, the logic chooses which data stream to use according to the device positions received by GOOSE. There are no wires to switch, but a new requirement appears: if the stream or the position is lost, the function must block instead of using the last known reference.

What happens if the replica is wrong

Voltage selection depends on the same auxiliary contacts used by the 87B replica and by the interlocks. A disconnector that reports its position wrongly affects all three at once. Typical symptoms are a close rejected without explanation on one bus and accepted on the other, or a constant angle difference that disappears when the bay changes bus. The most dangerous case gives no symptoms: the wrong reference happens to be in synchronism with the line on the day the breaker is closed. See disconnector replica for the 87B.

How it is tested

With secondary injection, different voltages are injected on each bus and each line, and the position of each disconnector is changed to check that the 25 always compares the right pair. In breaker-and-a-half and ring bus every possible topology is worked through, including maintenance ones. Blocking on invalid position and on loss of the selected voltage is also tested.

We configure it with you

Our team designs and configures the voltage selection logic, the 25 function and the disconnector replica in protective relays and bay controllers of the main brands, for high and extra-high voltage substations, industry and generating plants. See remote relay configuration and secondary engineering, or contact us.

References

IEEE C37.2, Standard for Electrical Power System Device Function Numbers, device 25.
IEC 61850-7-4, logical nodes RSYN, XSWI and XCBR.
IEC 61850-9-2, sampled values.
National Grid (NESO), technical specification for synchronizing and voltage selection (cited as an example of a utility specification).
J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions

Which voltage does the 25 use when the bay has both bus disconnectors closed?

During an on-load transfer the two buses are tied through the coupler and through the bay, so they have the same voltage. The selection logic usually gives priority to one of them by design, and what matters is that it never parallels the secondaries of the two bus voltage transformers.

Why must the secondaries of two voltage transformers never be paralleled?

Because if the buses are not tied on the primary side, the two secondary voltages can differ and a current circulates between the transformers that blows fuses or damages them. Even with the buses tied, paralleled secondaries hide the failure of one of the transformers. That is why selection schemes use switching with priority, not paralleling.

How is voltage selection solved in a digital substation?

When voltages arrive as sampled values over the process bus, selection stops being a relay that switches wires and becomes logic that chooses which data stream to use according to the device positions received by GOOSE. The principle is the same; the way it is implemented and tested changes.