Buchholz and Sudden Pressure Relays: Transformer Protection & NERC PRC-005-6

Part A — Buchholz Relays, Sudden Pressure Relays and PRC-005-6 at a Glance

Every liquid-filled power transformer carries two kinds of protection. Electrical protection — transformer differential (87T), restricted earth fault (87N), overcurrent (50/51) and thermal (49) — measures currents and voltages. Mechanical protection measures what the fault does to the oil: the gas it produces, the pressure wave it creates and the oil surge it drives. The Buchholz relay and the sudden pressure relay are the two classic mechanical devices, and for utility and generator transformers on the US Bulk Electric System, their trip functions fall under NERC Reliability Standard PRC-005-6 as Sudden Pressure Relaying.

Why mechanical protection still matters

A short circuit between a few turns of a large winding may change terminal currents by only a few percent — too little for a differential relay set with the slope and pickup needed for CT errors, tap changers and inrush. Yet the arc inside the tank decomposes oil into gas within milliseconds. A Buchholz relay or sudden pressure relay sees that gas or pressure directly. Mechanical relays also detect events with no electrical signature at all: core faults, loss of oil and slowly developing overheating.

The devices in one table

where Parc is the arc power. A 1 MW arc produces about 70 liters of gas per second — enough to drive oil violently toward the conservator and raise tank pressure.

3.3 Slow gas and dissolution

Slowly generated gas largely dissolves in the oil before reaching the Buchholz relay. Only when the oil near the fault becomes locally saturated, or the generation rate is high enough, do free bubbles rise and collect. This is why a Buchholz gas alarm indicates a fault that is already significant, and why online dissolved gas monitoring detects incipient faults much earlier.

4. The Buchholz relay

4.1 Construction

4.2 Operating sequence

4.3 Typical ratings and installation

Settings must be chosen with the transformer manufacturer. A setting too low can operate on oil movement when forced-oil pumps start or during through-faults; a setting too high reduces sensitivity.

4.4 Worked example: sensitivity floor of the surge element

The surge element can operate only if the oil velocity in the pipe reaches its setting. In steady state, the volume flow toward the conservator equals the gas generation rate:

For D = 80 mm, A = 0.00503 m², kg = 70 cm³/kJ and a setting of 1.0 m/s:

Below about 72 kW, the surge element does not operate no matter how long the arc persists; protection relies on the gas element, which needs free gas to collect, and on dissolved gas monitoring.

4.5 Worked example: time to gas alarm



If an incipient fault releases free gas into the relay at a given rate, the time to reach a 200 cm³ alarm volume is:

These rates refer to free gas actually arriving at the relay, which can be a small fraction of the gas generated at the fault.

5. Sudden pressure relays

5.1 Principle

A sudden pressure relay — the device NERC calls a fault pressure relay — responds to the rate of rise of pressure rather than to static pressure. A typical design uses a sensing bellows or diaphragm exposed to tank pressure on one side, and a reference chamber connected through a small equalizing orifice on the other. Slow pressure changes from temperature or load equalize through the orifice; a rapid rise produces a differential pressure that operates a microswitch.

The behavior can be approximated as a first-order lag:

The relay operates when the differential reaches its setting. For a constant rate of rise r, the differential approaches rτ, so the relay has an effective minimum operating rate:

Above this rate, operating time falls as the rate increases. Manufacturers publish operating curves of time versus rate of rise; Keentel's model uses illustrative values of a 7 kPa (about 1 psi) differential setting and a 2-second equalizing time constant, giving a minimum operating rate of about 3.5 kPa/s (0.5 psi/s).

5.2 Gas-space and under-oil relays

5.3 Worked example: pressure rise in a gas cushion



For a sealed tank with a gas cushion of volume Vg at absolute pressure p0, gas generation Q compresses the cushion approximately isothermally:

With p0 = 101.3 kPa, Vg = 3 m³ and a 1 MW arc producing 0.07 m³/s:

This is a relatively slow rate for a gas-space relay because the cushion absorbs the pressure, which is one reason under-oil relays respond faster. The real pressure wave near the fault is much faster and larger than this average estimate, because oil is nearly incompressible and the tank walls take time to deflect.



5.4 Misoperation risks

Sudden pressure relays have a mixed history of misoperation, which led many US utilities in past decades to use them for alarm only. Common causes include:

IEEE C37.91 and industry reports describe supervision schemes. A common approach requires the sudden pressure relay contact and an independent electrical indication — for example, an instantaneous overcurrent or differential element pickup, or a neutral current detector — before tripping. Supervision must be designed so that it does not block tripping for the low-level faults that the sudden pressure relay exists to detect; a supervision element set too high defeats the purpose.

5.5 Worked example: operating time versus rate of rise

For a constant rate of pressure rise r, the first-order model gives a differential pressure that builds as:

so the operating time for a setting Δpset is:

With Δpset = 7 kPa and τ = 2 s (illustrative values):

The curve explains the relay's two personalities: very slow and insensitive to gradual changes such as thermal expansion, and very fast for the steep pressure fronts of high-energy faults. When testing, the test kit must deliver a rate of rise above the minimum operating rate; a test at too low a rate will fail a good relay, and recording the applied rate is good practice. Actual characteristics differ by manufacturer and model, so procedures must use the manufacturer's published curve and test values.

5.6 Application checklist for sudden pressure relays

6. Keentel's surge and pressure model

6.1 Equations



The model represents the tank as a compliance C (combining oil compressibility and tank wall flexibility), the

with ρ = 870 kg/m³, L = 3 m, D = 80 mm, K = 3 and C = 5 × 10⁻⁷ m³/Pa.



6.2 Results and interpretation

Values are read from the model and rounded. They show the trends, not design values: the lower the surge setting, the more sensitive and faster the relay, but the greater the risk of operation on non-fault oil movement. Real transformers have more complex geometry, multiple oil paths and nonlinear tank deflection; manufacturer data and field experience govern settings.



The model's predicted tank pressures at high arc power — hundreds of kilopascals within a few hundred milliseconds — also illustrate why fast electrical clearing matters. Transformer tanks and pressure relief devices are designed for modest overpressure; a high-energy fault left uncleared for several cycles can rupture a tank and start a fire, which NFPA 850 and utility fire protection practices seek to prevent.

7. An integrated transformer protection scheme

7.1 Tripping design considerations



• Mechanical relay trips typically operate the transformer lockout relay (86T), which trips all breakers and blocks reclosing until reset.
• Breaker failure initiation from mechanical relays requires care, because breaker failure schemes usually rely on current detectors that may not pick up for low-level faults. Some designs use breaker auxiliary contact logic for these trips.
• Contacts on the transformer are exposed to harsh environments. Interposing auxiliary relays, sealed cable entries and trip circuit monitoring improve reliability.
• Event recording of mechanical relay operations, with time stamps, supports post-event analysis and misoperation investigation.

8. NERC PRC-005-6 in depth

8.1 Purpose and scope

PRC-005-6, "Protection System, Automatic Reclosing, and Sudden Pressure Relaying Maintenance," requires applicable entities to document and implement maintenance programs so that these systems remain in working order. Sudden Pressure Relaying was added in PRC-005-6, following FERC's directive to address it after recognizing that fault pressure relays, although not part of the defined Protection System, trip BES Elements and must be maintained.

8.2 Definitions

The Buchholz surge (trip) element and sudden pressure relays that trip BES Elements fall within this definition. Gas accumulation elements used only for alarm, and thermal or oil-level devices, do not.

8.3 Applicability

PRC-005-6 applies to Transmission Owners, Generator Owners and Distribution Providers for the systems that protect BES Elements. For transformers, this generally means Sudden Pressure Relaying that trips interrupting devices isolating BES transformers — for example, transmission transformers with primary and secondary terminals at 100 kV or above (such as 345/138 kV and 230/115 kV autotransformers) and generator step-up transformers of BES generating units. Dispersed generating resources are covered as specified in the standard. Each entity must confirm applicability against the BES definition and its registration.

8.4 Requirements

8.5 Table 5 — Sudden Pressure Relaying

Related components also apply: trip coils of the interrupting devices (Table 1-5, 6 calendar years), station dc supply (Table 1-4, with 4-month, 18-month and longer activities depending on battery type) and alarm paths (Table 2).

8.6 What "verify the sensing mechanism is operable" means in practice



The standard does not prescribe a test method. Common approaches:

Evidence should show the device, date, method, as-found and as-left condition, and the person performing the test.

8.7 Performance-based maintenance

Attachment A allows longer intervals based on demonstrated performance:

• define segments of similar Components with at least 60 Components in each segment;
• maintain on time-based intervals until at least 30 Components have results;
• set intervals so that Countable Events occur on no more than 4% of Components;
• maintain at least the greater of 5% of the segment or 3 Components each year;
• analyze results annually; if Countable Events reach 4% or more, implement an action plan to get below 4% within 3 years.

8.8 Worked example: performance-based segment

A utility owns 84 sudden pressure relays of the same make and model on BES transformers. Over the last two years it tested 30 relays and found 3 relays with failed microswitches or blocked orifices requiring replacement.

This exceeds the 4% threshold, so the utility cannot extend the interval. It must keep the segment on an interval that reduces Countable Events below 4% and implement an action plan, such as replacing the aging model. If, after replacement, testing finds 1 Countable Event in 32 relays (3.1%), the segment can support a performance-based interval, subject to annual testing of at least max(5% × 84, 3) = 5 relays.

8.9 Worked example: time-based compliance and VSL exposure

A transmission owner has 40 BES transformers with Sudden Pressure Relaying, each with one fault pressure relay, one lockout device and unmonitored control circuitry — 120 Components. During an audit, 7 Components are found past their maximum interval.

Under the R3 Violation Severity Levels, failing to maintain more than 5% but no more than 10% of Components is a Moderate VSL. Missing 6 Components (5.0%) would have been Lower. A robust maintenance tracking system with automatic warnings well before the maximum interval is the most effective mitigation.

8.10 Evidence and recordkeeping

8.11 Practical PSMP design for sudden pressure relaying

1. Inventory: list every fault pressure relay that trips a BES Element, with location, model, trip path and associated lockouts and breakers.
2. Classify: Component Types — fault pressure relay, lockout, control circuitry — and monitoring status.
3. Choose methods: time-based for small fleets; performance-based for large, uniform fleets.
4. Write procedures: test methods, equipment, safety, acceptance criteria and evidence forms.
5. Schedule: coordinate with transformer outages; many tests require isolating valves or de-energization.
6. Track: maintenance management system with due dates well ahead of maximum intervals.
7. Correct: document Unresolved Maintenance Issues and corrective actions.
8. Review: annual program review and lessons from misoperations.

8.13 Worked example: scheduling a 6-year Sudden Pressure Relaying program

A transmission owner with 42 BES transformers (each with one fault pressure relay and one lockout) wants an even workload. Tests require a transformer outage about half the time, so they must fit the outage plan.

Scheduling to complete each cycle in about five years — rather than at the six-year limit — leaves room for cancelled outages, storms and relay replacements. Combining fault pressure relay tests with trip coil (6-year) and dc system activities in the same outage reduces both cost and risk.



8.14 Relationship to other NERC standards

8.15 Status of revisions

PRC-005-6 is the effective version. A NERC project has been developing revisions (proposed PRC-005-7) focused mainly on clarifying applicability to protective functions in excitation systems and other equipment; entities should monitor the project for any changes that affect their programs.

9. Field testing and safety

9.1 Buchholz relay test procedure (typical)

1. Obtain clearances and verify the transformer state required by the procedure; many tests can be done energized only with alarm and trip outputs isolated, and only if the manufacturer and owner procedures allow.
2. Isolate trip outputs at the test switch, or schedule a transformer outage for full trip testing.
3. Inject dry air through the test cock until the upper float operates; record volume and alarm.
4. Release air and refill with oil; confirm the float resets.
5. Operate the surge element with the mechanical test device; confirm the trip contact.
6. Verify the trip path through interposing relays, lockout and breaker trip coils as required by the PSMP.
7. Inspect wiring, cable glands and terminal boxes for moisture and damage.
8. Restore and document.

9.2 Sudden pressure relay test procedure (typical)

1. Close the isolation valve between relay and tank.
2. Connect the test kit to the test port.
3. Apply a controlled pressure rise; confirm operation; record pressure and, where specified, time.
4. Confirm the relay does not operate for slow pressure changes, if the procedure includes this check.
5. Vent, reopen the isolation valve and confirm the relay is reset.
6. Verify the trip path as required.

9.3 Safety

Work on transformer protection involves energized equipment, stored energy in dc circuits and oil handling. NFPA 70E governs electrical safe work practices, OSHA 29 CFR 1910.269 applies to generation, transmission and distribution work, and the NESC governs clearances. Oil spills must be managed under spill prevention plans.

10. After an operation: diagnosis

10.1 Gas alarm

A Buchholz gas alarm means free gas has collected. Steps:

1. Note the volume of gas in the relay and the time since the last observation.
2. Collect a gas sample using a gas sampling device; do not ignite gas as a field test.
3. Collect an oil sample from the main tank for dissolved gas analysis per IEEE C57.104.
4. Compare free gas with dissolved gas using Ostwald coefficients.
5. Decide whether the transformer can remain in service, with increased monitoring, or must be removed.

10.2 Worked example: free gas versus dissolved gas

The equilibrium concentration of a gas in oil is approximately the gas-phase concentration times the Ostwald coefficient:

Using typical mineral oil Ostwald coefficients near 25 °C:

The measured dissolved gases are far below equilibrium with the free gas, meaning gas is being generated faster than it can dissolve — an active, energetic fault. The high acetylene and hydrogen content points to arcing. The transformer should be removed from service for diagnostic testing.

If instead the measured dissolved gases were close to the equilibrium values, the free gas would most likely have evolved slowly from oil that was already gassy, or from a historical event, and the urgency would be lower.

10.3 Trip

After a Buchholz surge or sudden pressure trip, the transformer must not be re-energized until the cause is understood. Typical diagnostics per IEEE C57.152 include dissolved gas analysis, turns ratio, winding resistance, excitation current, sweep frequency response analysis, insulation power factor and capacitance, leakage reactance, and core insulation resistance. A trip with no gas, normal electrical tests and evidence of a wiring fault or mechanical shock points to a misoperation; the cause must still be corrected.

11. Upgrades and modernization

12. Common misconceptions corrected

13. Key takeaways

1. Mechanical relays detect faults electrical relays can miss, especially low-level turn faults and core faults.
2. Understand the elements: gas accumulation alarm, oil surge trip, rate-of-rise pressure trip.
3. Correct the physics: floats drop as gas collects; the trip element responds to surge.
4. Know the limits: sensitivity floors, operating times and misoperation causes.
5. PRC-005-6 applies to trip functions on BES transformers: 6 years for relays and lockouts, 12 years for unmonitored control circuitry, or a qualified performance-based program.
6. Diagnose every operation: gas analysis, Ostwald comparison and field testing before re-energizing.

14. How Keentel Engineering helps

Keentel Engineering supports transmission owners, generator owners, renewable and storage developers and industrial facilities with transformer protection and NERC compliance, from 4 kV to 765 kV.

Why clients choose Keentel Engineering

• Protection and compliance under one roof: engineers who design protection and understand NERC standards.
• Practical field knowledge: test procedures that work in substations, not just on paper.
• Audit-ready documentation: programs and evidence structured for Regional Entity audits.
• Transformer expertise: from 13.8 kV distribution transformers to 765 kV autotransformers and generator step-up units.
• Licensed professional engineering: a Florida-registered firm with offices in Tampa, Austin, Sacramento and Baltimore, supporting projects nationwide.

Need a PRC-005-6 program review, sudden pressure relay testing procedures or transformer protection design? Talk to Keentel Engineering at (813) 389-7871, contact@keentelengineering.com, or book a 15-minute scoping call at calendly.com/keentel-engineering/15min.

Part C — Case Studies

The following are illustrative scenarios based on conditions typical of US utilities and generator owners. They show how Keentel Engineering approaches mechanical transformer protection and PRC-005-6 compliance; site details are generalized and figures are rounded.

Case Study 1 — 230/115 kV autotransformer: sudden pressure misoperation on an external fault

Situation. A transmission owner's 230/115 kV, 336 MVA autotransformer tripped by its sudden pressure relay during a close-in 115 kV line fault. The line fault was cleared correctly by line protection. The transformer showed no gas and passed initial electrical tests.

Analysis. Keentel reviews event records and finds that the sudden pressure relay operated about 80 ms after fault inception, during a through-fault of about 12 times transformer rating. The relay was an older model with no through-fault immunity, unsupervised, and installed close to a forced-oil pump discharge. Testing shows its operating rate threshold had drifted lower than the manufacturer's specification.

Engineered solution. Keentel recommends replacement with a current-generation under-oil sudden pressure relay, relocation away from the pump discharge, and supervision by a differential element pickup set low enough to preserve sensitivity for internal faults. The PSMP is updated to include the new relay and its supervision path.

Outcome. The transformer returns to service after diagnostics confirm no internal damage, and subsequent through-faults cause no further operations.

Case Study 2 — 345 kV generator step-up transformer: Buchholz alarm prevents a failure

Situation. A 345 kV generator step-up transformer at a combined-cycle plant receives a Buchholz gas alarm. Plant operators consider resetting the alarm because electrical protection shows no problem.

Analysis. Keentel advises gas sampling and dissolved gas analysis. Free gas is rich in hydrogen and acetylene, and dissolved gases are far below equilibrium with the free gas, indicating an active arcing source. Electrical testing during a planned outage shows abnormal winding resistance on one phase.

Engineered solution. Internal inspection finds a loose and burned lead connection. The connection is repaired, oil is processed, and an online dissolved gas monitor is installed with alarms to the control room.

Outcome. A likely in-service failure is avoided, and the plant gains early warning capability for the remaining life of the transformer.

Case Study 3 — Transmission owner: Sudden Pressure Relaying missing from the PSMP

Situation. During preparation for an audit, a transmission owner discovers that its PSMP covers protective relays, communications, sensing, dc supply and control circuitry, but not Sudden Pressure Relaying on 28 BES transformers.

Analysis. Keentel performs an inventory: 28 transformers, 31 fault pressure relays (some transformers have both Buchholz and sudden pressure trips), 28 lockouts already covered under protective relay control circuitry, and unmonitored trip paths. Several relays have no maintenance records in the past six years.

Engineered solution. Keentel drafts PSMP revisions defining Sudden Pressure Relaying Component Types and methods, writes test procedures for each relay model, and builds a prioritized test plan coordinated with outage schedules. Relays without records are tested first. The client also adds trip circuit monitoring on new installations to simplify future maintenance.

Outcome. The owner brings all Sudden Pressure Relaying within its maximum intervals before the audit, documents its corrective actions, and establishes a sustainable maintenance program.

Part D — Technical FAQ: Buchholz Relays, Sudden Pressure Relays and PRC-005-6

References and Further Reading

Links were current at publication (October 2026).



• NERC Reliability Standard PRC-005-6, Protection System, Automatic Reclosing, and Sudden Pressure Relaying Maintenance, and supporting Supplementary Reference and FAQ. North American Electric Reliability Corporation.
• NERC Glossary of Terms Used in NERC Reliability Standards (Protection System, Sudden Pressure Relaying, Countable Event, Unresolved Maintenance Issue). North American Electric Reliability Corporation.
• IEEE Std C37.91, Guide for Protecting Power Transformers. IEEE.
• IEEE Std C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations. IEEE.
• IEEE Std C57.104-2019, Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers. IEEE.
• IEEE Std C57.152, Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors. IEEE.
• IEEE Std C57.12.00, General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers. IEEE.
• NFPA 70E, Standard for Electrical Safety in the Workplace; NFPA 850, Fire Protection for Electric Generating Plants and High Voltage Direct Current Converter Stations. National Fire Protection Association.
• OSHA 29 CFR 1910.269, Electric Power Generation, Transmission, and Distribution.
• National Electrical Safety Code (NESC), IEEE C2. IEEE.
  

Disclaimer

his article is general technical information for educational purposes. It is not engineering, legal or compliance advice for any specific facility and does not create a professional relationship. Keentel Engineering's oil-surge and pressure model is a simplified illustration with assumed parameters; actual relay settings, operating times and test methods must follow manufacturer data, transformer design and owner procedures. NERC requirements are summarized; entities must consult the current effective standard, their registration and their Regional Entity, and maintain their own compliance programs. Designs and procedures must follow applicable IEEE standards, the NESC, NFPA codes and OSHA requirements, sealed by a licensed professional engineer where required.

Case studies are illustrative scenarios based on typical project conditions, not records of specific client projects. Figures are rounded and indicative.



NERC, IEEE, NFPA and other names are the property of their respective owners. Keentel Engineering is not affiliated with or endorsed by these organizations or any equipment vendor.