7.4 SFRA, Sudden-Pressure Relay & Dry-Transformer ATS Tests

Sweep Frequency Response Analysis (SFRA)

Sweep Frequency Response Analysis (SFRA) is a diagnostic test that detects mechanical displacement of a transformer's core and windings — damage that is invisible to TTR, IR, and winding resistance tests. It is governed by IEEE Std C57.149 (Guide for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers).

The test set applies a low-voltage sinusoidal signal swept across a frequency range — typically 20 Hz to 2 MHz — to one end of a winding and measures the response at the other end. The ratio of output to input voltage (the transfer function) is plotted versus frequency. The resulting trace is a fingerprint of the winding's distributed resistance, inductance, and capacitance (RLC) network.

How the Trace Is Interpreted

A measured trace is compared against a baseline fingerprint (taken at the factory or on initial commissioning) or against a sister unit (an identical transformer tested at the same time). Deviations in specific frequency bands point to specific defects:

For a Dyn1 transformer, IEEE C57.149 recommends a set of nine tests: three HV open-circuit, three LV open-circuit, and three HV-to-LV short-circuit measurements. The test is most informative when a baseline exists; without one, a sister-unit comparison is the next best option, and a single one-off measurement can only flag gross damage.

When to Run SFRA

SFRA is run after any event that could have mechanically damaged the transformer:

• After shipment or transport (especially over rough roads or long distances)
• After a through-fault (a fault on the external system that the transformer survived)
• After an internal fault (confirmed or suspected)
• As part of a major overhaul or before re-energizing a long-stored unit

It is often paired with a leakage reactance test (IEEE C57.152). A leakage reactance deviation greater than +/- 3% from nameplate warrants investigation, and the per-phase measurements should compare within +/- 3% of their average.

Sudden-Pressure Relay (SPR)

A sudden-pressure relay (SPR), also called a fault-pressure relay, is a mechanical protection device mounted on liquid-filled transformer tanks. It detects a rapid pressure rise inside the tank caused by internal arcing or gassing faults — the kind of fault that produces gas quickly enough to raise tank pressure before a dissolved-gas analysis sample would catch it.

The relay's trip output goes to an 86 lockout relay, which trips the transformer's source and lockout out re-energization until an operator investigates. The SPR is sensitive to the rate of pressure rise, not steady pressure, so normal thermal expansion and slow gas generation do not trip it.

How the SPR Is Tested

Field testing is functional: a calibrated pressure-injection kit (or hand pump with a quick-acting valve) introduces a small, rapid pressure pulse into the relay's sensing chamber. The technician verifies that the relay contacts close and that the 86 lockout operates. The test confirms mechanical freedom and electrical continuity; it does not verify the exact pressure-rise trip threshold, which is set at the factory.

SPR False-Trip Sources

• Oil pump surges — starting a forced-oil pump can create a pressure transient that trips an older or mis-set SPR.
• Inrush — heavy magnetizing inrush on energization can move enough oil to trigger a marginal relay.
• Tap-changer operation — OLTC tap changes create small pressure pulses.
• Cooling system transients — sudden temperature changes from cooling fans or radiators.

When investigating an SPR operation, check the event log for inrush, tap changes, and pump starts before condemning the transformer.

Dry-Type Transformer Tests

Dry-type transformers use air or cast-resin insulation, not oil. Their acceptance test suite per ANSI/NETA ATS 7.2 is shorter than a liquid-filled unit's:

Visual and mechanical inspection on a dry-type unit focuses on anchoring, corona shields, ventilation clearances, and the condition of the encapsulated windings. Factory impulse tests (BIL) are not normally repeated in the field.

ATS-Related Transformer Tests

When a transformer is part of an automatic transfer scheme (ATS) or emergency power system, the Level 2 technician verifies that the transformer behaves correctly during the transfer:

• Inrush energization — when the ATS transfers to the transformer, the transformer draws magnetizing inrush of 6 to 10 times full-load current for the first few cycles, decaying over seconds. The downstream breaker must be sized to ride through inrush, and the protective scheme must not trip on it. The acceptance test verifies that the transformer energizes without tripping and that inrush does not nuisance-trip upstream or downstream devices.
• Load tap changer (LTC) cycling — if the transformer has an LTC, the test cycles the tap changer through its range (often with the transformer de-energized for resistance checks, then energized for functional checks) and verifies continuity at every tap. A dynamic-resistance or winding-resistance check across the tap range catches worn diverter-switch contacts.
• Transfer timing — the ATS transfer time must be coordinated with the transformer's inrush and the downstream load ride-through capability, particularly for NFPA 110 Type 10 systems (10-second transfer).

NETA Level 2 depth means the technician knows how to perform these tests under supervision, record the results, and recognize out-of-tolerance conditions — not design the protection scheme.