Gas-actuated protection devices play an important role in protecting oil-immersed power transformers against internal faults. When an internal fault occurs inside a transformer, the heat generated by the fault can decompose transformer oil and insulation materials. This decomposition produces gases and may cause a sudden increase in pressure inside the transformer tank.
If this pressure is not detected or controlled in time, it can damage the transformer tank, cause oil leakage, or even lead to a serious transformer failure.
To protect transformers against such conditions, different types of gas and pressure-operated protection devices are used. The three important devices are:
- Pressure Relief Device (PRD)
- Rate of Rise Pressure Relay (Sudden Pressure Relay)
- Buchholz Relay
Although all three devices are associated with internal transformer faults and pressure or gas generation, their operating principles and functions are different.
This article explains their construction, working principles, applications, testing, advantages and limitations in simple language.
1. Introduction: Why Are Gas Actuated Devices Required in Transformers?
A power transformer is filled with insulating oil, which performs two important functions:
- It provides electrical insulation between live parts and the transformer tank.
- It carries heat away from the transformer windings and core.
During normal operation, transformer oil remains stable under the designed operating conditions. However, when an internal electrical fault occurs, a large amount of heat may be produced at the fault location.
This heat can cause the following:
- Decomposition of transformer oil.
- Breakdown of paper and other insulating materials.
- Formation of combustible gases.
- Sudden expansion of oil and gases.
- Increase in pressure inside the transformer tank.
For example, an inter-turn fault in a winding may produce a high temperature at a small location. Similarly, an earth fault or phase-to-phase fault may produce an electrical arc, resulting in rapid gas generation.
An electrical arc may have an arc voltage of approximately 50–200 V. The amount of gas generated depends on the fault energy, oil condition, fault location and other factors. The reference material gives an approximate gas generation range of 50–200 cm³ per kW·s.
If the gas generation and pressure rise are not detected, the transformer may suffer serious internal damage.
2. Types of Gas Actuated Devices Used in Transformers
The main devices used for detecting gas generation and abnormal pressure conditions are described below.
| Device | Main function | Typical action |
|---|---|---|
| Pressure Relief Device (PRD) | Releases excessive pressure from the transformer tank | Opens mechanically to release pressure |
| Rate of Rise Pressure Relay | Detects a rapid increase in internal pressure | Operates electrical alarm or trip contacts |
| Buchholz Relay | Detects gas accumulation and oil movement between the main tank and conservator | Gives alarm or trips the transformer |
It is important to understand that these devices do not perform exactly the same function. A pressure relief device releases excessive pressure, a sudden pressure relay detects a rapid pressure rise, and a Buchholz relay detects gas accumulation and oil movement.
3. Pressure Relief Device (PRD)
3.1 What Is a Pressure Relief Device?
A Pressure Relief Device (PRD) is a mechanical safety device installed on an oil-filled transformer tank. Its main purpose is to release excessive internal pressure when the pressure inside the tank exceeds a predetermined value.
During a serious internal fault, gases may be generated rapidly due to the decomposition of transformer oil and insulation. This can cause a sudden increase in tank pressure.
If the pressure becomes too high, the transformer tank may deform or rupture. The pressure relief device helps prevent such damage by releasing the excessive pressure.
Depending on the design, a pressure relief device may also be provided with electrical contacts to initiate an alarm or trip signal.
3.2 Construction of a Pressure Relief Device
A typical pressure relief device consists of the following parts:
- Valve Body: The main housing of the device, which is mounted on the transformer tank.
- Valve Seat: The surface against which the valve closes to prevent oil or gas from escaping during normal operation.
- Spring: Holds the valve in its closed position under normal pressure conditions.
- Valve Disc: Opens when the internal pressure produces sufficient force to overcome the spring force.
- Alarm Contacts: Electrical contacts provided in certain designs to indicate that the device has operated.
- Protective Cover: Protects the operating mechanism from external environmental conditions.
The exact construction may vary depending on the manufacturer and model.
3.3 Working Principle of a Pressure Relief Device
The pressure relief device works on the principle of pressure force acting against spring force.
Under normal operating conditions, the spring keeps the valve closed. When an internal fault occurs, gas generation and oil expansion may increase the pressure inside the transformer tank.
The device operates as follows:
During normal operation, the pressure inside the transformer tank remains within the permitted range. The spring keeps the valve disc firmly seated, so oil and gas cannot escape.
When an internal fault occurs, such as an inter-turn fault, earth fault or phase-to-phase fault, the fault energy produces heat. This heat decomposes transformer oil and generates gas.
The generated gas and expanding oil increase the internal pressure of the transformer tank.
When the pressure force acting on the valve disc becomes greater than the spring force, the valve starts opening.
The valve opens and releases excessive gas and/or oil from the transformer tank through the pressure relief outlet. This reduces the internal pressure and helps protect the tank from mechanical damage.
If electrical indication contacts are provided, the operation of the device can initiate an alarm or trip command through the associated control and protection circuit.
After operation, the device may require manual resetting, depending on its design. The cause of operation must be investigated before the transformer is returned to service.
3.4 Pressure Relief Valve and Pressure Relief Relay
The terms pressure relief valve and pressure relief device are sometimes used together, but it is useful to distinguish their functions.
- Mechanical pressure relief: Opens to release excessive pressure from the transformer tank.
- Electrical indication: Auxiliary contacts, where provided, indicate operation and may initiate an alarm or trip.
A PRD is not the same as a sudden pressure relay. A PRD generally responds when pressure reaches its mechanical operating threshold, whereas a sudden pressure relay responds to the rate at which pressure increases.
3.5 Testing and Maintenance of Pressure Relief Devices
Regular inspection and testing help ensure that the pressure relief device remains in proper working condition.
Important maintenance activities include:
- Check the device body for physical damage, corrosion and oil leakage.
- Inspect the valve and its mounting arrangement.
- Check the condition of the alarm or trip contacts, if provided.
- Verify the associated control wiring and indication.
- Check the reset mechanism according to the manufacturer's instructions.
- Confirm that the discharge path is not obstructed.
- Investigate the cause whenever the device operates.
4. Rate of Rise Pressure Relay (Sudden Pressure Relay)
4.1 What Is a Rate of Rise Pressure Relay?
A Rate of Rise Pressure Relay, also known as a Sudden Pressure Relay (SPR), is a protective device used to detect a rapid increase in gas or oil pressure inside an oil-filled transformer tank.
Its main purpose is to detect sudden pressure changes caused by internal faults.
Unlike a conventional pressure-operated device that responds mainly to a particular pressure level, a sudden pressure relay responds to how quickly the pressure increases.
For example:
- A gradual pressure increase due to normal temperature changes may not operate the relay.
- A sudden pressure increase due to an internal electrical fault may operate the relay.
The relay is designed to distinguish between normal slow pressure changes and abnormal rapid pressure changes.
4.2 Device Number of Sudden Pressure Relay
According to ANSI/IEEE device-number terminology, Device 63 is associated with a pressure relay.
In some protection drawings and manufacturer documents, additional suffixes such as 63S or 63SPR may be used to identify a sudden pressure relay. Other suffixes may also appear in particular drawings or equipment documentation.
The exact designation should always be confirmed from the transformer protection schematic and manufacturer's documentation.
4.3 Construction of a Rate of Rise Pressure Relay
A typical sudden pressure relay consists of the following components:
1. Pressure-Sensing Diaphragm or Bellows
The diaphragm or bellows is the main sensing element. It responds to changes in pressure inside the transformer tank.
When a sudden pressure increase occurs, the diaphragm or bellows moves and operates the switching mechanism.
2. Equalizing Orifice (Bleed Hole)
An equalizing orifice is a small opening that allows pressure to equalize gradually across the sensing element.
It plays an important role in distinguishing between slow and sudden pressure changes.
3. Microswitch or Contact Assembly
The movement of the diaphragm or bellows operates a microswitch or electrical contact assembly. The contacts are connected to the transformer alarm or trip circuit as designed.
4. Housing and Mounting Flange
The housing protects the internal mechanism. The mounting flange provides mechanical installation on the transformer tank.
5. Test and Reset Arrangement
Depending on the design, a test or reset arrangement may be provided to check the operation of the relay and restore it after operation.
4.4 Working Principle of a Rate of Rise Pressure Relay
The working principle of a sudden pressure relay is based on the rate of change of pressure.
The equalizing orifice allows slow pressure changes to balance gradually. However, when the pressure increases suddenly, the equalizing orifice cannot balance the pressure quickly enough. As a result, the diaphragm or bellows moves and operates the electrical contacts.
The working can be understood in the following steps.
During normal transformer operation, pressure may change slowly due to temperature variations, oil expansion and contraction, or normal operating conditions.
The equalizing orifice allows these gradual pressure changes to equalize across the sensing element. Therefore, the relay normally remains in its reset position.
Suppose an internal electrical fault occurs inside the transformer winding.
The fault may produce an electrical arc, resulting in rapid heating and decomposition of transformer oil and insulation materials.
The decomposition of oil and insulation may produce gases such as:
- Hydrogen (H₂)
- Methane (CH₄)
- Ethylene (C₂H₄)
- Acetylene (C₂H₂)
The gas generation and rapid expansion create a sudden pressure increase inside the transformer tank.
The pressure rises faster than it can equalize through the small bleed orifice.
Consequently, a pressure difference develops across the diaphragm or bellows.
The pressure difference causes the sensing element to move.
The movement operates the microswitch or electrical contact mechanism.
The relay sends an electrical signal to the associated protection circuit. Depending on the design and protection scheme, it may initiate an alarm or trip the transformer circuit breaker.
4.5 Why Does a Sudden Pressure Relay Not Operate During Slow Pressure Changes?
This is one of the most important points to understand.
The relay contains an equalizing orifice that allows slow pressure changes to balance across the diaphragm.
For example, when transformer oil temperature increases gradually, the internal pressure may also increase gradually. The pressure can equalize through the orifice, so the diaphragm does not move sufficiently to operate the contacts.
However, during a serious internal fault, pressure may increase very rapidly. The orifice cannot equalize the pressure immediately. A pressure difference develops across the diaphragm, causing the relay to operate.
| Condition | Pressure behaviour | Relay response |
|---|---|---|
| Normal temperature variation | Slow pressure change | Normally does not operate |
| Gradual oil expansion | Slow pressure change | Normally does not operate |
| Internal electrical fault | Rapid pressure increase | Operates when its design threshold is reached |
| Sudden internal arcing | Very rapid pressure increase | May initiate a trip command |
4.6 Sensitivity of a Sudden Pressure Relay
The sensitivity of a sudden pressure relay depends on its design and manufacturer.
The reference material gives an illustrative operating sensitivity of approximately:
This is approximately:
These values should not be treated as universal settings for every transformer. The actual operating characteristics must be obtained from the relay manufacturer's technical data.
4.7 Types of Sudden Pressure Relay
Based on the location of the pressure-sensing element, sudden pressure relays may be classified into two types.
1. Gas-Space Sudden Pressure Relay
A gas-space sudden pressure relay is installed in the gas space above the transformer oil level.
It is generally used in transformers having a gas blanket or sealed-tank arrangement.
It detects rapid pressure changes in the gas space.
2. Oil-Space Sudden Pressure Relay
An oil-space sudden pressure relay is installed in a position where it senses pressure changes in the transformer oil.
It detects rapid pressure changes transmitted through the oil.
The choice between gas-space and oil-space arrangements depends on transformer construction, pressure-relay design and manufacturer recommendations.
4.8 Applications of Sudden Pressure Relay
Sudden pressure relays may be used in:
- Large oil-filled power transformers.
- Transformers with sealed-tank or gas-blanket arrangements.
- Transformers having separate oil compartments for on-load tap changers (OLTCs).
- Shunt and series reactors.
- Phase-shifting transformers.
- Certain large oil-filled instrument transformers, where specified by the manufacturer.
The application depends on equipment design and the protection philosophy. A particular transformer capacity should not be treated as a universal requirement for installing this relay.
4.9 Installation and Maintenance of Sudden Pressure Relay
Correct installation is important for reliable operation.
Important points include:
- Install the relay in the location specified by the transformer manufacturer.
- Follow the correct mounting orientation.
- Use the appropriate gas-space or oil-space arrangement.
- If an isolation valve is provided, ensure that it is fully open during normal service.
- Check the electrical wiring and associated trip or alarm circuit.
- Verify proper earthing and cable shielding wherever specified.
- Inspect the relay for physical damage, corrosion and leakage.
Testing activities may include:
- Visual inspection of the relay.
- Checking electrical contact continuity.
- Checking alarm and trip circuit wiring.
- Operating the test mechanism, if provided.
- Performing a controlled pressure test using the manufacturer's prescribed procedure, where applicable.
- Checking insulation resistance where required by the applicable maintenance procedure.
- Confirming correct reset and contact operation.
Periodic maintenance intervals must follow the manufacturer's recommendations and the utility's approved maintenance schedule. The reference material mentions an interval of 1–3 years, but this should not be considered a universal requirement.
4.10 Advantages of Sudden Pressure Relay
- It detects rapid pressure increases caused by certain internal transformer faults.
- It can operate quickly when the pressure rise meets its operating characteristics.
- It may detect some internal arcing faults before the pressure reaches the mechanical operating threshold of a pressure relief device.
- It does not depend on current or voltage measurement to detect the pressure event.
- Suitable designs can be used in sealed transformers without a conservator.
4.11 Limitations of Sudden Pressure Relay
- It cannot identify the exact location or type of internal fault.
- It does not provide a gas sample for chemical analysis.
- Its mechanical sensing mechanism may require inspection and testing.
- Its performance depends on correct installation and manufacturer-specific operating characteristics.
- It does not replace differential protection, Buchholz protection or a pressure relief device.
5. Buchholz Relay
5.1 What Is a Buchholz Relay?
A Buchholz Relay is a gas-actuated protective device used in oil-immersed transformers fitted with a conservator tank.
It is installed in the connecting pipe between the main transformer tank and the conservator.
Its main purpose is to detect internal faults by sensing:
- Gas accumulation caused by the decomposition of transformer oil and insulation.
- Sudden oil movement caused by a serious internal fault.
The Buchholz relay provides two-stage protection:
- First stage: Alarm due to gas accumulation.
- Second stage: Trip due to severe oil movement.
It is one of the important protective devices used in conservator-type oil-filled transformers.
5.2 Why Is a Buchholz Relay Required?
When an internal fault occurs inside a transformer, heat is generated at the fault location.
This heat decomposes the transformer oil and insulating materials, producing gases.
The amount and rate of gas generation depend on the severity and nature of the fault.
For example:
- A minor insulation fault may produce gas slowly.
- A loose connection may cause local heating.
- An inter-turn fault may produce gas due to overheating.
- A serious internal arc may generate a large quantity of gas rapidly and cause a sudden movement of oil.
The Buchholz relay detects these abnormal conditions and provides an alarm or trip signal.
5.3 Location of Buchholz Relay
The Buchholz relay is installed in the connecting pipe between the main transformer tank and the conservator tank.
The connecting pipe is arranged so that gas generated inside the main tank can travel towards the conservator through the relay chamber.
The relay is generally installed with the orientation and pipe inclination specified by the transformer and relay manufacturers.
5.4 Construction of Buchholz Relay
A typical Buchholz relay consists of the following parts:
1. Metallic Chamber
The relay has a metallic chamber installed in the connecting pipe between the transformer tank and conservator.
The chamber contains the operating mechanism and floats.
2. Upper Float
The upper float is positioned near the top of the relay chamber.
When gas accumulates inside the chamber, the oil level falls and the upper float moves downward.
This movement operates the alarm contact.
3. Lower Float or Baffle Arrangement
The lower operating element is provided to detect sudden oil movement.
During a serious internal fault, a rapid flow of oil from the main tank towards the conservator moves the lower float or baffle mechanism.
This operates the trip contact.
4. Electrical Contacts
The relay contains electrical contacts connected to the transformer alarm and trip circuits.
Older relay designs may use mercury switches, while modern designs commonly use magnetic, reed or other electrical contact arrangements.
5. Gas Sampling Cock
A gas sampling arrangement is provided to collect gas accumulated inside the relay chamber.
The collected gas can be tested to help identify the nature of the internal fault.
6. Inspection Window
Some designs have an inspection window through which the operating mechanism or gas accumulation can be visually checked.
5.5 Working Principle of Buchholz Relay
The Buchholz relay works on the principle of gas accumulation and oil surge.
It provides two-stage protection.
First Stage: Gas Accumulation – Alarm Operation
The first stage operates when a small amount of gas accumulates inside the Buchholz relay chamber.
The operation is explained below.
A minor internal fault, such as local overheating, insulation deterioration or a loose connection, may occur inside the transformer.
The heat produced by the fault decomposes transformer oil and insulation material, generating gas.
The gas moves upwards through the connecting pipe and enters the Buchholz relay chamber.
As gas accumulates at the upper portion of the chamber, it displaces the transformer oil.
The oil level inside the relay gradually falls.
When the oil level falls, the upper float moves downwards.
The movement of the upper float operates the alarm contact.
An alarm is then received in the control room or through the associated annunciation system.
What does this alarm indicate?
A Buchholz alarm indicates that gas has accumulated inside the relay chamber. It may be due to an internal fault, but the gas must be examined before deciding the exact cause.
Gas accumulation should never be ignored because it may be an early indication of transformer insulation deterioration or an internal fault.
Second Stage: Oil Surge – Trip Operation
The second stage operates when a serious internal fault produces a rapid movement of oil from the main tank towards the conservator.
The operation is explained below.
A severe internal fault, such as a phase-to-phase fault, winding-to-earth fault or major internal arcing fault, occurs inside the transformer.
The fault produces intense heat, causing rapid decomposition of transformer oil and insulation.
The rapid gas generation causes a sudden increase in pressure inside the transformer tank.
The pressure increase produces a rapid movement of oil through the connecting pipe towards the conservator.
The fast-moving oil strikes the lower operating element of the Buchholz relay.
The lower float or baffle mechanism moves and operates the trip contact.
The trip contact sends a signal to the associated protection circuit, which operates the circuit breaker and disconnects the transformer from the power system.
This prevents further damage to the transformer.
5.6 Difference Between First-Stage and Second-Stage Operation
| Particular | First Stage | Second Stage |
|---|---|---|
| Operating element | Upper float | Lower float or baffle |
| Main cause | Gas accumulation | Sudden oil movement |
| Fault condition | Usually minor or developing fault | Usually severe internal fault |
| Operation | Alarm | Trip |
| Purpose | Warns about abnormal gas generation | Disconnects the transformer |
5.7 Gas Analysis in Buchholz Relay
One important advantage of the Buchholz relay is that it allows the collection of gas accumulated inside its chamber.
The collected gas can be examined to understand the possible nature of the internal fault.
Gas generation occurs because transformer oil and insulation materials decompose when exposed to abnormal heating or electrical discharge.
The gases commonly associated with transformer faults include:
| Gas | Chemical formula |
|---|---|
| Hydrogen | H₂ |
| Methane | CH₄ |
| Ethane | C₂H₆ |
| Ethylene | C₂H₄ |
| Acetylene | C₂H₂ |
| Carbon monoxide | CO |
| Carbon dioxide | CO₂ |
Different gases may be produced under different fault conditions. For example, hydrogen may be associated with several types of electrical and thermal faults, while acetylene is commonly associated with high-energy arcing.
The gas composition can provide useful information about the condition of the transformer.
However, gas composition alone should not be used to declare the exact fault location or cause. It should be evaluated along with other test results and transformer operating history.
Dissolved Gas Analysis (DGA) is commonly used to examine gases dissolved in transformer oil. Gas collected from a Buchholz relay can also be analysed as part of fault investigation.
5.8 Gas Decomposition and Temperature
The reference material indicates that decomposition of transformer oil begins at approximately 350°C.
This value should be understood as an approximate reference rather than a fixed temperature for every gas-generation process. Gas formation depends on the oil composition, insulation material, duration of heating, electrical discharge and other conditions.
5.9 Installation Guidelines for Buchholz Relay
Correct installation is essential for reliable operation of a Buchholz relay.
The following installation points are important:
1. Correct Location
The relay must be installed in the connecting pipe between the main transformer tank and the conservator.
2. Pipe Inclination
The connecting pipe must have the inclination specified by the manufacturer so that gas can travel towards the conservator through the relay chamber.
The exact angle should be taken from the approved transformer drawing. The angle stated in the older reference material is unclear and should not be used as an installation setting without verification.
3. Straight Pipe Length
The reference material gives the following straight-pipe requirements:
- Tank-to-relay straight length: more than 5D.
- Relay-to-conservator straight length: more than 3D.
Here, D represents the internal diameter of the connecting pipe.
These values should be checked against the applicable relay manufacturer's instructions and approved transformer drawings before installation.
4. Nominal Pipe Bore
The reference material gives the following pipe sizes according to transformer rating:
| Transformer capacity | Nominal pipe bore |
|---|---|
| Up to 1000 kVA | 25 mm |
| 1000–10,000 kVA | 50 mm |
| Above 10,000 kVA | 80 mm |
These are reference values, not universal requirements. The actual pipe size must follow the transformer design and applicable specifications.
5. Correct Direction of Installation
The relay must be installed in the direction indicated by the manufacturer. The arrows or flow-direction markings on the relay should be followed.
6. Gas Sampling Arrangement
The gas sampling cock should be accessible for inspection and collection of gas samples.
5.10 Advantages of Buchholz Relay
- It detects gas accumulation caused by developing internal faults.
- It provides an early alarm before some faults become severe.
- It provides a trip signal during serious internal faults involving rapid oil movement.
- It allows gas collection for fault investigation.
- It can help identify developing transformer insulation problems.
- It provides two-stage protection through separate alarm and trip functions.
5.11 Limitations of Buchholz Relay
Although the Buchholz relay is an important protective device, it has certain limitations.
1. Applicable to Conservator-Type Transformers
A conventional Buchholz relay requires a connecting pipe between the main tank and conservator. Therefore, it is not normally used in sealed transformers without a conservator arrangement.
2. Detects Faults Inside the Oil-Filled Tank
It detects gas generation and oil movement associated with faults within the protected oil-filled tank.
It does not provide protection for external connecting cables or other external equipment. Separate protection is required for those parts.
3. Cannot Detect Every Type of Transformer Fault
Faults that do not produce sufficient gas or oil movement may not operate the Buchholz relay.
4. Operating Time Depends on Fault Conditions
The relay response depends on gas generation, oil movement, fault severity and relay characteristics. The reference material mentions a minimum operating time of 0.1 second, but this should not be treated as a universal operating time for all Buchholz relays.
5. Requires Regular Inspection
The relay, contacts, wiring and gas sampling arrangement must be inspected and maintained according to the approved maintenance schedule.
5.12 Buchholz Relay in OLTC
An On-Load Tap Changer (OLTC) is used to regulate transformer output voltage by changing the effective winding turns while the transformer is in service.
During tap changing, switching operations occur inside the OLTC compartment. These operations may produce gases and oil movement due to the switching process.
For this reason, some transformer designs provide a separate protective arrangement for the OLTC oil compartment.
Where specified by the manufacturer, a separate Buchholz-type or oil-surge relay may be installed for OLTC protection.
A Buchholz relay installed on the main transformer tank does not automatically protect a separate OLTC oil compartment. The OLTC requires its own protection arrangement according to its design.
6. Difference Between Pressure Relief Device, Sudden Pressure Relay and Buchholz Relay
All three devices are associated with internal transformer faults, but their operating principles and functions are different.
| Particular | Pressure Relief Device | Sudden Pressure Relay | Buchholz Relay |
|---|---|---|---|
| Main function | Releases excessive tank pressure | Detects rapid pressure increase | Detects gas accumulation and oil surge |
| Operating principle | Pressure force overcomes spring force | Rate of pressure rise | Gas accumulation and oil movement |
| Installation | On transformer tank | On tank at the location specified by the design | Between main tank and conservator |
| Main sensing element | Spring-loaded valve | Diaphragm or bellows | Floats or baffle mechanism |
| Response | At a specified mechanical pressure threshold | At a specified pressure-rise characteristic | Due to gas accumulation or oil surge |
| Alarm | If auxiliary contacts are provided | If configured in the protection scheme | First-stage operation |
| Trip | If connected and designed for trip | Commonly connected to trip circuit | Second-stage operation |
| Pressure release | Yes | No | No |
| Gas sampling | No | No | Yes |
| Application | Oil-filled transformer tanks | Suitable oil-filled transformer designs | Conservator-type oil-filled transformers |
6.1 Difference Between Sudden Pressure Relay and Pressure Relief Device
This difference is particularly important.
A Sudden Pressure Relay detects how quickly pressure increases inside the transformer tank. It operates electrical contacts to initiate an alarm or trip command according to the protection scheme.
A Pressure Relief Device opens mechanically when the internal pressure reaches its designed operating threshold. It releases excessive gas and/or oil to reduce tank pressure.
Therefore:
- Sudden pressure relay = Detects rapid pressure rise.
- Pressure relief device = Releases excessive pressure.
Both devices may be installed on the same transformer because they perform different protective functions.
6.2 Difference Between Buchholz Relay and Sudden Pressure Relay
A Buchholz relay is installed in the connecting pipe between the transformer main tank and conservator. It detects gas accumulation and sudden oil movement.
A sudden pressure relay is installed on the transformer tank at a manufacturer-specified location. It detects rapid changes in internal pressure.
The Buchholz relay provides gas accumulation alarm and oil-surge trip functions, whereas a sudden pressure relay generally initiates an electrical alarm or trip based on its design and protection connections.
7. Coordination with Other Transformer Protection Devices
Gas-actuated devices are not intended to replace the complete transformer protection system. They work together with other protective relays.
Some important transformer protection devices include:
| Protection device | ANSI device number | Main function |
|---|---|---|
| Transformer Differential Protection | 87 | Detects internal electrical faults by comparing currents |
| Overcurrent Relay | 50/51 | Detects excessive current |
| Earth Fault Protection | 50N/51N or 64, depending on scheme | Detects earth-fault conditions |
| Buchholz Relay | 63 | Detects gas accumulation and oil surge |
| Sudden Pressure Relay | 63, with suffixes where specified | Detects rapid pressure rise |
| Winding Temperature Protection | 49 or 26, depending on scheme | Protects against excessive temperature |
| Pressure Relief Device | No single universal ANSI number for the mechanical valve function | Releases excessive tank pressure |
The actual protection arrangement depends on transformer rating, design, utility standards and the manufacturer's recommendations.
9. Frequently Asked Questions (FAQs)
Q1. What are gas-actuated devices in a transformer?
Gas-actuated devices are protective devices that detect gas generation, oil movement or abnormal pressure conditions caused by internal transformer faults. Important examples include the Buchholz relay and sudden pressure relay. A pressure relief device provides mechanical protection against excessive tank pressure.
Q2. What is the main function of a pressure relief device?
The main function of a pressure relief device is to release excessive internal pressure from the transformer tank to help prevent tank deformation or rupture.
Q3. What is the working principle of a sudden pressure relay?
It works on the principle of rate of pressure rise. It detects rapid pressure changes caused by internal faults while allowing gradual pressure changes to equalize through an orifice.
Q4. Why is an equalizing orifice provided in a sudden pressure relay?
The equalizing orifice allows slow pressure changes to balance across the sensing element. During a sudden pressure increase, pressure cannot equalize quickly enough, causing the diaphragm or bellows to move and operate the contacts.
Q5. Where is a Buchholz relay installed?
A Buchholz relay is installed in the connecting pipe between the main transformer tank and the conservator.
Q6. Why does a Buchholz relay have two floats?
The upper float is generally used for gas accumulation and alarm operation. The lower float or baffle is used to detect sudden oil movement and initiate transformer tripping.
Q7. What does the Buchholz alarm indicate?
A Buchholz alarm indicates that gas has accumulated inside the relay chamber and displaced the oil. This may indicate a developing internal fault and requires investigation.
Q8. What does the Buchholz trip indicate?
A Buchholz trip generally indicates a severe internal fault that has caused rapid oil movement through the relay. The relay operates its trip contact to disconnect the transformer.
Q9. Can a Buchholz relay be installed in a sealed transformer?
A conventional Buchholz relay is normally used in a conservator-type transformer because it is installed in the connecting pipe between the main tank and conservator. A sealed transformer without this arrangement requires a different protection design.
Q10. What is the difference between a Buchholz relay and a sudden pressure relay?
A Buchholz relay detects gas accumulation and oil surge in the connecting pipe, whereas a sudden pressure relay detects a rapid pressure increase inside the transformer tank.
Q11. Does a pressure relief device trip the transformer?
The primary function of a pressure relief device is mechanical pressure release. Some designs also have auxiliary contacts that can initiate an alarm or trip signal through the protection circuit.
Q12. Can the Buchholz relay detect faults in a separate OLTC compartment?
Not automatically. If the OLTC has a separate oil compartment, it may require its own protective relay or oil-surge protection arrangement as specified by the manufacturer.