Buchholz Relay MCQ Quiz - Objective Question with Answer for Buchholz Relay - Download Free PDF

Buchholz relay

• Buchholz relay is an oil and gas actuated relay located in the pipeline connecting the transformer main tank and the conservator.
• It provides early warning and protection against internal faults such as short circuits, insulation failures, and overheating by detecting gas accumulation and oil flow disturbances within the transformer tank.
• The use of a Buchholz relay for the protection of oil-immersed transformers is generally considered uneconomical for transformers with a rating of less than 750 kVA. This is because smaller transformers are less prone to internal faults that require gas accumulation detection, and alternative protection methods (such as fuses and circuit breakers) are more cost-effective for lower ratings.

Operation:

1. In case of incipient faults within the transformer, the heat due to the fault causes the decomposition of some transformer oil in the main tank. The products of decomposition contain more than 70% of hydrogen gas. The hydrogen gas, being light, tries to go into the conservator and in the process gets entrapped in the upper part of the relay chamber. When a predetermined amount of gas gets accumulated, it exerts sufficient pressure on the float to cause it to tilt and close the contacts of the mercury switch attached to it. This completes the alarm circuit to sound an alarm.
2. If a serious fault occurs in the transformer, an enormous amount of gas is generated in the main tank. The oil in the main tank rushes towards the conservator via the Buchholz relay and in doing so tilts the flap to close the contacts of mercury switch. This completes the trip circuit to open the circuit breaker controlling the transformer

Explanation:

Differential Protection in Transformers

Definition: Differential protection is a type of protective relay scheme used to protect electrical transformers from internal faults. It operates on the principle of comparing the difference between the currents entering and leaving the transformer. If the difference exceeds a pre-set threshold, it indicates an internal fault, and the protection system isolates the transformer to prevent damage.

Working Principle: The differential protection scheme utilizes current transformers (CTs) placed on both the primary and secondary sides of the transformer. These CTs measure the current flowing into and out of the transformer. Under normal operating conditions, the current entering the transformer should equal the current leaving it, considering the transformer’s turns ratio. Any significant difference between these currents indicates an internal fault. The protection relay detects this discrepancy and trips the circuit breaker, isolating the transformer from the network.

Advantages:

• Provides fast and accurate protection against internal faults.
• Reduces the risk of severe damage to the transformer and associated equipment.
• Enhances the reliability and stability of the power system by minimizing the duration of faults.

Disadvantages:

• Requires precise current transformers with accurate matching characteristics.
• Can be complex and expensive to implement, especially for large transformers.

Applications: Differential protection is widely used in power transformers, generators, and large motors to ensure the equipment's safety and reliability. It is particularly crucial in critical infrastructure and industrial applications where transformer failures can lead to significant disruptions and economic losses.

Correct Option Analysis:

The correct option is:

Option 1: Internal ground faults.

This option correctly describes the primary purpose of differential protection in transformers. Internal ground faults, or any internal fault within the transformer, cause a difference in the current entering and leaving the transformer. The differential protection scheme detects this discrepancy and isolates the transformer to prevent damage.

Important Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Temperature rise.

Temperature rise in a transformer is typically monitored by thermal protection schemes, not differential protection. Thermal protection uses temperature sensors to detect overheating conditions and activate cooling systems or trip the transformer if necessary to prevent thermal damage.

Option 3: Gas formation.

Gas formation within a transformer, often indicative of internal arcing or insulation breakdown, is detected using gas detectors like Buchholz relays. These relays sense the accumulation of gas within the transformer tank and provide an early warning or trip the transformer to prevent catastrophic failure.

Option 4: Failure of cooling system.

The failure of the cooling system in a transformer leads to overheating, which is monitored by thermal protection devices. While it is critical to address cooling system failures, differential protection schemes are not designed to detect such issues.

Conclusion:

Understanding the different protection schemes and their specific applications is essential for ensuring the safety and reliability of electrical transformers. Differential protection is a specialized and highly effective method for detecting and isolating internal faults within transformers, thereby preventing severe damage and maintaining the stability of the power system. While other protection methods address issues like temperature rise, gas formation, and cooling system failures, differential protection remains crucial for safeguarding transformers against internal electrical faults.

Basic principles of transformer protection

• Gas protection
• Longitudinal differential protection or current quick-break protection
• Backup protection for phase-to-phase short circuit
• Zero sequence protection of grounding short circuit
• Overexcitation protection

​Explanation 

• The Buchholz relay is normally not provided in small distribution transformers.
• Buchholz relay, however, is a gas-actuated protection device that is typically used in oil-filled transformers of larger sizes.
• It detects internal faults like short circuits and insulation breakdowns by sensing gas accumulation in the oil. This type of protection is generally not provided in small distribution transformers due to their simpler design and lower cost.
• Overfluxing protection and overcurrent protection are commonly provided in transformers, including small distribution transformers, to protect against overvoltage and excessive current conditions, respectively.

Concept:

Buchholz's relay:

• A Buchholz relay is a safety device that protects oil-filled transformers and reactors from internal faults and short circuits
• Buchholz relay is a protection device which is normally used in large oil absorbed transformers; It is a kind of oil and gas actuated relay.
• Whenever a small fault happens within the electrical device, heat is generated by the fault currents
• The generated heat causes decomposition of electrical device oil and gas bubbles are generated
• These gas bubbles run in the upward direction and obtain collected within the Buchholz relay
• The collected gas relocates the oil in Buchholz relay and therefore the displacement is similar to the amount of gas collected
• The dislocation of oil causes the higher float to shut the higher mercury switch to connect an alarm circuit
• Hence, once a small fault happens, then the alarm will be activated; The collected quantity of gas specifies the level of fault occurred
• Different types of faults have different types of oil flow and thus a fault can be recognized easily, and proper action can be taken

Differential relay:

• Differential relay operation depends on the phase difference of two or more electrical quantities.
• It works on the principle of comparison between the phase angle and the magnitude of the same electrical quantities.
• The differential relay is used for the protection of the feeder, large busbars, etc.

​

Distance relay:

• Distance relay is widely used for the protection of high-voltage AC transmission line and distribution lines.
• Distance protection schemes are commonly employed for providing the primary or main protection and backup protection for AC transmission line and distribution line against three-phase faults, phase-to-phase faults, and phase-to-ground faults.

Under frequency relay:

• Under frequency relay is used to protect the alternator when the frequency drops below the operating frequency.
• It is a backup protection for over fluxing (V/F) protection.
• Under frequency occurs due to turbine low speed, grid frequency fluctuation, etc.
• The generator can tolerate moderate under frequency operation provided voltage is within an acceptable limit.

Buchholz relay:

• Buchholz relay protects the transformer from internal faults.
• It is the gas actuated relay.
• The Buchholz relay is placed between the main tank and the conservator.
• Buchholz relay is used in the transformer having a rating higher than 500KVA.
• It is not used in a small transformer because of economic considerations.

Conservator:

• Transformer oil is used for the purpose of insulation and cooling.
• The oil should not be allowed to come in contact with atmospheric air, as it may spoil its insulating properties.
• To prevent this, we use conservators.
• Conservator consists of an airtight metal drum fixed above the level of the top of the oil tank and connected with it by a pipe.
• The function of a conservator is to take up contraction and expansion of oil without allowing it to come in contact with outside air.

Note:

Breather: The function of a breather in the transformer is to filter out the moisture from the air. Breather consists of silica gel which absorbs the moisture from the air.

Buchholz Relay: Buchholz relay is used for the protection of transformers from the faults occurring inside the transformer.

Buchholz Relay:

• The Buchholz relay protects the transformer from internal faults
• It is a gas actuated device
• It is placed between the main tank and the conservator
• It is used in the transformer has a rating higher than 500 KVA
• It causes alarm for minor fault and tripping for a major fault
• Buchholz relay has capability of anticipating the possible major fault in a transformer.

• Silica gel is used to absorb moisture and prevent entering the oil tank while breathing. It is used in an oil transformer breather.
• The Colour of fresh silica gel is blue. The moist silica gel became pink in color.
• A transformer consists of Breather, Conservator and Buchholz relay, etc.
• The breather is used in the transformer to filter out the moisture from the air. Breather consists of silica gel which absorbs moisture from the air.
• Conservator tank present at the top of the transformer which allows adequate space for expansion of oil. Therefore during an overloading condition, the oil moves to the conservator tank.
• Buchholz relay is used for the protection of transformers from the faults occurring inside the transformer.
• Whenever the transformer is loaded, the temperature of the transformer insulating oil increases. Consequently, the volume of oil is increased.
• As the volume of the oil is increased, the air above the oil level in conservator will come out. At low oil temperature, the volume of the oil is decreased, causes air to enter into a conservator tank.
• The air consists of moisture in it and this moisture can be mixed up with oil. Hence to filter the air from a moisture silica gel breather is used.

• Silica gel is used to absorb moisture and prevent entering the oil tank while breathing. It is used in an oil transformer breather.
• The Colour of fresh silica gel is blue. The moist silica gel became pink in color.
• A transformer consists of Breather, Conservator and Buchholz relay, etc.
• The breather is used in the transformer to filter out the moisture from the air. Breather consists of silica gel which absorbs moisture from the air.
• Conservator tank present at the top of the transformer which allows adequate space for expansion of oil. Therefore during an overloading condition, the oil moves to the conservator tank.
• Buchholz relay is used for the protection of transformers from the faults occurring inside the transformer.
• Whenever the transformer is loaded, the temperature of the transformer insulating oil increases. Consequently, the volume of oil is increased.
• As the volume of the oil is increased, the air above the oil level in conservator will come out. At low oil temperature, the volume of the oil is decreased, causes air to enter into a conservator tank.
• The air consists of moisture in it and this moisture can be mixed up with oil. Hence to filter the air from a moisture silica gel breather is used.

Breather, Conservator and Buchholz relay is part of the transformer but commutator is a part of D.C machine.

Breather: The function of a breather in the transformer is to filter out the moisture from the air. Breather consists of silica gel which absorbs the moisture from the air.

Conservator: There is a conservator tank at the top of the transformer which allows adequate space for expansion of oil. Therefore during overloading condition, the oil moves to the conservator tank.

Explanation:

• Silica gel is used to absorb moisture and prevent entering the oil tank while breathing. It is used in an oil transformer breather.
• The Colour of fresh silica gel is blue. The moist silica gel became pink in color.
• A transformer consists of Breather, Conservator and Buchholz relay, etc.
• The breather is used in the transformer to filter out the moisture from the air. Breather consists of silica gel which absorbs moisture from the air.
• Conservator tank present at the top of the transformer which allows adequate space for expansion of oil. Therefore during an overloading condition, the oil moves to the conservator tank.
• Buchholz relay is used for the protection of transformers from the faults occurring inside the transformer.
• Whenever the transformer is loaded, the temperature of the transformer insulating oil increases. Consequently, the volume of oil is increased.
• As the volume of the oil is increased, the air above the oil level in conservator will come out. At low oil temperature, the volume of the oil is decreased, causes air to enter into a conservator tank.
• The air consists of moisture in it and this moisture can be mixed up with oil. Hence to filter the air from a moisture silica gel breather is used.

Buchholz relay is used for the protection of transformers from the faults occurring inside the transformer.

Short circuit faults such as inter-turn faults, incipient winding faults and core faults may occur due to the impulse breakdown of the insulating oil or simply the transformer oil.

• Buchholz relay is a protection device which is normally used in large oil absorbed transformers; It is a kind of oil and gas activated relay
• Whenever a small fault happens within the electrical device, heat is generated by the fault currents
• The generated heat causes decomposition of electrical device oil and gas bubbles are generated
• These gas bubbles run in the upward direction and obtain collected within the Buchholz relay
• The collected gas relocates the oil in Buchholz relay and therefore the displacement is similar to the amount of gas collected
• The dislocation of oil causes the higher float to shut the higher mercury switch to connect an alarm circuit
• Hence, once a small fault happens, then the alarm will be activated; The collected quantity of gas specifies the level of fault occurred
• Different types of faults have different types of oil flow and thus a fault can be recognised easily, and proper action can be taken

Buchholz relay:

Buchholz relay is a gas actuated relay and it is used to protect the transformer against all internal faults.

Key Points

• It is a gas actuated device
• It is placed between the main tank and the conservator
• It is used in the transformer has a rating higher than 500 KVA
• It causes alarm for minor fault and tripping for a major fault