Transformer differential protection (87T) — why it is fast, but does not replace the Buchholz relay

The guide on the Buchholz relay covers a mechanical protection that reacts to gas formation and oil flow. This article covers the primary electrical protection of a power transformer against internal faults: percentage differential protection, in practice often referred to by the ANSI code 87T.

The basic principle: current in must equal current out

A differential relay compares, via current transformers (CTs) on both the primary and secondary side of the transformer, the current entering the transformer with the current leaving it. In a healthy transformer, with no internal fault, this is — after correction for the transformer's transformation ratio — essentially the same power going in on one side and out on the other; the difference between the two CT currents (the differential current) is then virtually zero. If an internal fault occurs — for example a short circuit between windings or a breakdown to the core — part of the current leaves the transformer through the fault itself rather than through the outgoing terminals, and a significant differential current arises.

Why a percentage of the restraint current, not a fixed threshold

A differential relay does not assess the differential current against a fixed ampere value, but against a percentage of the average through-flowing current (the "restraint" current). This is necessary because even a healthy transformer always shows a small, normal differential current — for example due to CT measurement errors, the transformer's own magnetising current, or an unequal tap-changer position load. At low through-flowing current the relay must be sensitive enough to detect a small, low-energy internal fault; at high through-flowing current (for example during a heavy but external, through-flowing short circuit) the relay must instead be less sensitive, because CT measurement errors and saturation effects grow larger. This declining sensitivity margin is represented as a restraint characteristic with typically two slopes: a lower slope (typically on the order of 15–30%) at moderate through-flowing current, and a higher slope (typically on the order of 50–80%) at heavy through-flowing current, where CT saturation starts to play a larger role.

Why both CT ratio and vector group must be compensated

Two practical complications mean that a differential relay cannot simply compute "current A minus current B":

Unequal CT transformation ratios: the primary and secondary rated current of a transformer typically differ substantially, so the current transformers on each side typically have a different transformation ratio. The relay (or, in older electromechanical designs, interposing transformers in the relay circuit) must compensate for this ratio before the two currents can be meaningfully compared.
Phase shift from the vector group: as covered in the guide on transformer vector groups, a Dyn11 transformer, for example, introduces a 30° phase shift between primary and secondary current. Without correcting for that phase shift, the relay would already see an apparent differential current in a fully healthy transformer. In electromechanical relays this correction was done via the winding configuration of the interposing transformers (often delta-connected on the side where the main transformer is star, and vice versa); in modern digital relays this is done computationally in the relay software, based on the configured vector group.

Second-harmonic blocking: inrush current is not an internal fault

The guide on transformer inrush current covers why energising an unloaded transformer can draw a brief, high magnetising current (inrush). Seen only from the primary side, this inrush current resembles a differential current, because the secondary side does not yet supply a corresponding current. To prevent the relay from tripping incorrectly on every energisation, a differential relay includes second-harmonic blocking: inrush current contains a characteristically high proportion of second harmonic (100 Hz on a 50 Hz network), while a real internal fault does not. If the proportion of second harmonic in the differential current exceeds a set threshold (often on the order of several tens of a percent), the relay blocks the trip, even though the differential current itself would otherwise be high enough to trigger it.

Why this does not make the Buchholz relay redundant

A percentage differential relay is considerably faster than a Buchholz relay and detects faults that already produce a measurable current imbalance as they arise. A slowly developing, low-energy fault — for example incipient partial discharge or gradually deteriorating insulation between a few windings — often does not yet produce, at an early stage, a current difference large enough to trip the differential protection, while the fault is already producing gas. This is precisely the type of fault that the Buchholz relay (see the related guide) detects via gas accumulation, well before the fault develops into an electrically detectable short circuit. Differential protection and Buchholz protection are therefore complementary, not interchangeable: one is fast and sensitive to current imbalance, the other is slow but sensitive to a type of low-energy fault the first can miss.

Note: the exact setting of the restraint slopes and the second-harmonic blocking threshold follows from the system study and the relay manufacturer's specification; this article covers the principle, not a specific setting value for a specific relay make.

Practical relevance

When assessing the protection concept of a power transformer, it is important to recognise that differential protection and the Buchholz relay complement each other: differential protection catches fast, current-intensive internal faults, while the Buchholz relay catches the slow, low-energy faults that do not yet produce a significant current imbalance. A transformer with only differential protection therefore lacks part of the protection coverage a Buchholz relay provides (and vice versa).

Common mistakes

Assuming a correctly functioning differential protection makes the Buchholz relay redundant — both protect against a different type of fault and are complementary.
Not correctly configuring the CT transformation ratio or vector-group compensation when commissioning a digital differential relay, so that a healthy transformer already shows an apparent differential current.
Disabling or setting the second-harmonic blocking too loosely to "solve" energisation nuisance tripping — this can actually reduce sensitivity to a real internal fault occurring at the moment of energisation.
Thinking the differential protection needs to be equally sensitive at low through-flowing current as at high through-flowing current — the restraint characteristic is specifically designed to become less sensitive at high through-flowing current, because of growing CT measurement errors.