Secondary-Selective: Main-Tie-Main Configuration

What a secondary-selective configuration is

It is a double-ended switchboard: two bus sections, each fed by its transformer through a main breaker, and a tie breaker between the two sections. That is why it is also called main-tie-main. Each transformer is fed from a different medium-voltage circuit. IEEE Std 141 (the Red Book) includes it among the basic industrial distribution configurations.

In the diagram of this series on substation configurations there are two medium-voltage sources, Transformer 1 and Transformer 2, the switchboard with Section 1 and Section 2, the tie and two groups of loads, one on each section. The same arrangement is used with medium-voltage switchgear fed by two high-voltage transformers.

How it operates in normal service

Both mains are closed and the tie is open. Each section works as an independent radial system with its transformer, and the short-circuit level on each section is that of a single transformer. Interlocking prevents closing all three breakers at once when the design does not allow paralleling. The logic of each device is in interlocking in secondary-selective systems, and the sequence can be practiced in the switching simulator.

Some designs operate with the tie closed so that there is no interruption when a source is lost. The cost is a higher short-circuit level in the switchboard, because the contributions of both transformers add up. With two equal transformers and a medium-voltage network impedance that is small compared with that of the transformers, it is about double. In addition, protection is needed to prevent one transformer from feeding a fault on the other.

What happens during faults and maintenance

• Loss of a supply or fault in a transformer. The transformer protection trips its medium-voltage breaker and its main. The tie is then closed, manually or automatically, and the section is fed by the other transformer.
• Fault on a bus section. The main of that section opens. Transfer must be blocked so that the tie is not closed onto the fault. The loads on that section are lost; the other section continues.
• Fault on a load or low-voltage feeder. Its breaker opens; nothing else notices if coordination is correct.
• Breaker failure (50BF). If a load breaker fails to open, the main of its section opens or, with the tie closed, the tie and the main. If the one that fails is the tie in single-ended operation, the whole switchboard is lost.
• Maintenance. A transformer, its cable or its main can be maintained with its section fed through the tie. Maintaining the tie or a bus section forces that section out of service.

Reliability, flexibility, cost and footprint

It is chosen when the process cannot wait for a transformer to be repaired. Compared with a primary-selective system, it also covers a transformer failure, at the price of duplicating the transformation capacity. Compared with a radial system, it costs more: it duplicates the transformation and adds the tie and its logic. Compared with a spot network, it has a brief interruption on transfer, but it needs no network protectors and does not operate with transformers in parallel. At high and medium voltage, the equivalent is the sectionalized single bus with two incoming lines. For more detail on how a spot network operates and is protected, read spot network.

Protection and control implications

• Three-level coordination. Load, tie and main must coordinate in time and current, with phase and ground overcurrent or with the electronic trip units of the low-voltage breakers. Coordination is checked with the tie open and closed.
• Automatic transfer. It starts on loss of voltage on a section, detected with undervoltage 27 on the VT on the transformer side, confirms voltage on the other section and opens the main before closing the tie. It is blocked if the main opened on overcurrent, because that indicates a bus fault.
• Transfer with motors. A technical bulletin from the NIH Office of Research Facilities (USA) describes four schemes: slow (fixed time), residual voltage, in-phase and fast. It cites ANSI/NEMA C50.41, which limits the resultant volts per hertz between the motor and the new source, at the instant of reconnection, to 1.33 pu.
• Paralleling with the tie closed. If allowed, the incoming breakers need protection that discriminates the direction of current, such as directional 67 or reverse power 32, so that a fault in one transformer is not fed by the other through the tie.
• Ground fault in four-wire switchboards. With a distributed neutral, part of the neutral current circulates between sections; ground fault protection of a double-ended switchboard needs a scheme designed for that.
• Transformers. 87T on large units, overcurrent and their own protections on the rest.

Typical use

At low voltage it is the double-ended switchboard of industrial plants, hospitals and data centers. At medium voltage it is used in industrial substations with two power transformers. The related equipment is covered in the guide on low-voltage switchboards and MCCs and in medium-voltage switchgear.

Commissioning tests

• Interlocks between mains and tie in all combinations, mechanical and electrical.
• Transfer and return logic with secondary injection: loss of voltage, timing, blocking on a bus fault and residual-voltage or phase criterion.
• Coordination of load, tie and main with the tie open and closed.
• Switchboard ground fault protection with current injected on each section.

Interlocking logic in gates

These are the conditions that must be met to enable operation of the key devices in this configuration, drawn with IEC 60617-12 logic gates (AND «&», OR «≥1»). An intermediate position or an invalid quality signal counts as «not met». The interlocking table generates the diagram for each device and command, and in the switching simulator you can test each condition.

We can help with your secondary-selective system

Transfer, blocking and three-level coordination are where these switchboards most often fail. At Tiza Group we carry out the protection coordination study for industrial plants and for medium-, high- and extra-high-voltage substations, remotely and worldwide.

Related guides

• Automatic transfer at medium voltage: logic and blocking
• Fast bus transfer for motor buses: three methods
• Spot Network: How It Operates and How It Is Protected

References

• IEEE Std 141 (Red Book): recommended practice for electric power distribution for industrial plants.
• IEEE Std 242 (Buff Book): protection and coordination of industrial and commercial power systems.
• IEEE Std 493 (Gold Book): design of reliable industrial and commercial power systems.
• ANSI/NEMA C50.41: Polyphase Induction Motors for Power Generating Stations (transfer and reclosing of motor buses).
• NIH, Office of Research Facilities, Technical Bulletin Secondary Selective Power Distribution Automatic Transfer, October 2014.

Frequently asked questions