T-Tap Substation Configuration: Operation and Protection

What the T-tap or tapped line configuration is

In the T configuration, a load, a transformer or a line section is connected to an existing line at an intermediate point, between its two substations, with no breakers sectionalizing the line at that point. On the single-line diagram the line is the crossbar of the T and the tap is the vertical stroke. It is also called a tapped line or tap.

In the tap there is usually, from the line toward the load:

• A tap disconnect switch, sometimes with a grounding blade, to isolate it without cutting the line.
• A transformer switching element: circuit breaker, switch-disconnector or fuses, depending on the voltage level and the rating.
• The tapped transformer, with its current transformers (CTs) on the breaker or in the bushings.

If the tap has generation or is connected to another network, it stops being a tapped load and becomes a three-terminal line, with protection at each end. If the tap is only load with no breaker or CTs, the relays at the line ends do not measure its current.

With two lines and two transformers, the diagram of this series shows each transformer tapped from a different line, with its disconnect switch and its high-side breaker.

How it operates under normal conditions

The line is closed between its two substations and the tap draws its load from the connection point. The relays at the ends measure the line current, but the tap current only appears as the difference between what enters at one end and what leaves at the other.

What happens during faults and maintenance

• Line fault. The breakers at both ends open and the tap is left without supply, even if the fault is far from it. If the load has motors or generation, it must be disconnected before reclosing.
• Fault in the tapped transformer with its own breaker. Its protection opens the tap breaker and the line stays in service, as long as the line protection does not operate first.
• Fault in the transformer with no breaker. The transformer protection must send direct transfer trip to both ends, and the whole line trips.
• Breaker failure (50BF). If the tap breaker fails to open, the backup is to open the line: transfer trip to both ends or, with no channel, clearing by the backup zones at the ends with a longer delay.
• Maintenance. The tap breaker is maintained with the transformer out of service. Any maintenance of the line, or of the breakers at its ends with no transfer bus, leaves the tap without supply.

Reliability, flexibility, cost and footprint

It is the cheapest configuration and the one with the smallest footprint, because it needs no breakers on the line. In exchange, the tap depends entirely on the line, and the line is exposed to the faults of the tap. Compared with a single bus with line in and line out, it loses the ability to sectionalize the line at that point. When the load grows, the usual step is to convert the tap into an in-and-out substation, for example in an H configuration.

Implications for protection and control

• Distance. The contribution or draw of the tap changes the apparent impedance seen from the ends and modifies the reach of the zones of the distance protection 21. A tapped transformer with a grounded-wye winding on the high side contributes zero-sequence current on ground faults, which the relays do not measure and which alters the zero-sequence voltage drops; the ground compensation factor must be reviewed carefully.
• Line differential 87L without measurement at the tap. The tap load current, faults on the low-voltage side of the transformer, its magnetizing inrush on energization and the zero sequence contributed by its grounded wye appear as false differential current. They are compensated with higher pickup and slope settings, supervision by distance or ground elements, or with signals from the tap, all at the cost of sensitivity and speed.
• Transfer trip. When a fault in the transformer or in the tap breaker must open the line, a communications channel is needed from the tap to both ends.
• Reclosing (79). The dead time must be longer than the time it takes to disconnect the load or generation at the tap; if the tap has generation, it needs its own anti-islanding protection.
• 89 interlocks. The tap disconnect switch operates without load, with the tap breaker open, and its grounding blade only with the disconnect switch open and no voltage on the transformer side. The logic of each device is in interlocks in a T configuration.

Typical use and grid-code limits

It is used in distribution and subtransmission to supply industrial or rural loads without building a switching substation, and as a temporary connection while the permanent one is built.

For reliability and security of the system, some grid codes do not allow T-connections of any type of customer to the transmission grid, and on distribution systems any connection must meet the technical standards and be approved by the utility. Always check the current text of the local code and the utility’s requirements; the design and the relay settings are adapted to them.

Commissioning tests

• End-to-end tests of the line with the tap simulated: load, low-voltage fault on the transformer and external ground fault.
• Transfer trip from the tap to both ends.
• Energization of the tapped transformer with the line in service, verifying that the 87L and the distance protection do not operate on magnetizing inrush.
• Transformer protections, tap breaker and disconnect switch interlocks.

See end-to-end testing and secondary injection.

Interlocking logic in gates

These are the conditions that must be met to enable operation of the key devices of 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 interlock table generates the diagram for each device and command.

We can help with your tap

A tap changes the relay settings at both ends of the line, not just those of the load. At Tiza Group we carry out the protection coordination study for medium-, high- and extra-high-voltage lines and substations, in utilities and in industry.

References

• IEEE C37.113: guide for protective relay applications to transmission lines (includes lines with more than two terminals).
• IEEE C37.119: guide for breaker failure protection.
• IEEE C37.104: guide for automatic reclosing of circuit breakers on transmission and distribution lines.
• IEEE C37.91: guide for protecting power transformers.
• IEC 61936-1: power installations exceeding 1 kV AC.
• B. Kasztenny, I. Voloh, M. Adamiak, J. Zeek, Application of Current Differential Protection to Tapped Transmission Lines, GE Power Management.

Frequently asked questions