H Configuration: Two Lines, Two Transformers and a Tie

What the H configuration is

The H configuration serves a substation with two lines and two transformers. Each line arrives at a node, a transformer leaves each node, and the two nodes are joined by a cross section or tie. Seen on the single-line diagram, it forms an H: the two vertical legs are line-transformer and the crossbar is the tie. This way either line can feed either transformer.

There are several variants, distinguished by how many breakers they have:

• Full H, with five breakers: one per line, one per transformer and one in the tie. It is the most flexible.
• H with three breakers: two on the lines and one in the tie, with the transformers connected to the node by disconnector. A transformer fault forces the breaker of its line and the tie to open.
• H with three breakers on the transformer side: two on the transformers and one in the tie, with the lines entering by disconnector.
• Tie with disconnectors instead of a breaker, which are only operated without load.

The International Electrotechnical Vocabulary (IEC 60050-605) does not use the name “H”, but it defines equivalent arrangements. One is the four-switch mesh. Another is its variant for, normally, two lines and two transformers, in which the fourth breaker is replaced by a link with one or two disconnectors. Each breaker has its disconnectors, each line its disconnector with an earthing blade and its voltage transformer (VT), and the current transformers (CT) are located at each breaker to overlap the zones.

How it operates in normal service

There are two usual modes. Tie closed: the two lines and the two transformers are in parallel. If the substation is tapped into a line, power can pass from one line to the other through the tie. Tie open: the substation works as two independent supplies, each line with its transformer. The tie is closed by command or by an automatic transfer scheme when a line is lost. The choice depends on short-circuit levels, the flow between the lines and the utility’s philosophy.

What happens on faults and maintenance

For the full five-breaker H:

• Line fault. Its protection trips the breaker of that line and the remote end trips its own. With the tie closed, the transformer on that branch stays fed by the other line.
• Transformer fault. The 87T transformer differential trips its high-side and low-side breakers. The two lines and the other transformer stay in service.
• Node fault. Each node is a small bus between the line breaker, the transformer breaker and the tie breaker. It is protected with its own zone or covered by overlapping zones, depending on the design, and clearing it trips the three breakers around it.
• Breaker failure (50BF). If a line breaker fails on a line fault, the tie and the transformer breaker of that branch trip. If the tie fails, the breakers of the opposite branch, line and transformer, trip, and half the substation is lost. In the variants without a line breaker, the backup is completed with direct transfer trip to the remote end.
• Maintenance. With a line breaker out, that line goes out of service, unless a bypass disconnector exists. With the tie out, the substation is left as two independent branches. With a transformer breaker out, that transformer goes out.

In the three-breaker variants, a fault in a transformer without its own breaker is cleared by opening the line of its branch and the tie, or by sending a transfer trip to the remote end if there is no local breaker.

Reliability, flexibility, cost and area

The full H offers more than a sectionalized single bus with two incomers. It has one more breaker than a four-breaker ring, and in return a transformer fault opens only that transformer’s breaker, without touching the lines or the tie. The three-breaker variants are cheaper than the ring, at the cost of losing a line or a transformer on faults that the ring would clear without cutting other circuits. Its great limitation is expansion: a third line forces a change of configuration. When the substation may grow, it is advisable to leave the yard ready to move to a ring or a double bus.

Compared with a T tap, the H is the next step when the load already justifies sectionalizing the line with breakers in the substation.

Implications for protection and control

• Zones. Each line with its protection (distance or differential) and its VT on the line side; each transformer with its 87T and its mechanical protection; the tie with its own backup 50/51 and its 50BF. Zones must overlap at each breaker.
• Variants without a transformer breaker. The 87T takes the CTs of the line breaker and the tie, or those of the transformer bushing, and its trip opens those two breakers.
• Automatic transfer. If the tie works open, the logic for closing on loss of a line lives in the bay controller or in a PLC, with verification of absence of fault, voltage and synchronism check 25 if both lines can be energized.
• Reclosing (79). Each line has a single local breaker, so reclosing is the classic one for a single breaker, blocked by the transformer protection.
• 89 interlocks. Disconnectors with their breaker open, earthing blades without voltage and, if there is a bypass disconnector, interlocks that prevent paralleling a line with its breaker open. The logic of each device is in interlocks in the H configuration, and the sequence can be practiced in the switching simulator (available in Spanish).

Typical use

It is used in subtransmission and distribution substations fed by two lines, and in industrial plants with two high-voltage incomers and two transformers. Whether it is accepted for a given connection depends on the local code and the utility’s requirements.

Tests at commissioning

• Ratio, polarity and excitation of each CT, and checking the stability of the 87T with load current.
• 50BF matrix of each breaker, especially that of the tie.
• Automatic transfer logic with loss of each line and with a fault present.
• Reclosing of each line and interlock matrix.

See secondary injection and power circuit breaker tests.

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 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 H substation

The H is simple in the yard, but the logic of the tie and the transfer defines how much service is lost on each fault. At Tiza Group we carry out the protection coordination study for medium-, high- and extra-high-voltage substations, in utilities and in industry. Studies are done remotely, worldwide.

References

• IEC 60050-605: International Electrotechnical Vocabulary, substations (terms 605-01-21 and 605-01-22).
• IEC 61936-1: power installations exceeding 1 kV a.c.
• IEEE C37.91: guide for protecting power transformers.
• IEEE C37.119: guide for breaker failure protection of power circuit breakers.
• IEEE C37.234: guide for protective relay applications to power system buses.
• CIGRE, Technical Brochure 161: General guidelines for the design of outdoor AC substations.
• J. D. McDonald (ed.), Electric Power Substations Engineering, CRC Press.

Frequently asked questions