Analysis of Transformer Gas Relay Action under Short Circuit Impact

A gas relay is an important non-electric protection device for transformers. Under the action of vibration caused by sudden short circuit impact, misoperation of gas relay leads to transformer tripping accidents frequently. In order to explore the relationship between the vibration intensity of the gas relay and the anti-vibration performance of the gas relay during a sudden short circuit, the sudden short circuit capacity test of the transformer and the vibration test of the gas relay were carried out. The analysis of test data shows that the sudden short circuit process of the transformer will cause the misoperation of some gas relays, which provides data support for the inspection test of gas relays. Finally, the optimization scheme of the detection method of the gas relay is proposed, which further effectively reduces the possibility of misoperation and improves the operation reliability of the transformer equipment.


Introduction
Power transformers are important equipment in power substations, and their safe and stable operation is crucial for the reliability of the power grid.Long-term operation of transformers can result in electrical and thermal stresses that cause a decrease in insulation levels.Additionally, external short circuit surges can cause a large amplitude of short circuit surge current to flow through the winding, generating significant electromagnetic forces that can lead to mechanical instability and transformer short circuit faults [1,2].Sudden short circuit faults are the most significant threat to transformer safety.According to statistics from the State Grid Corporation of China, power transformers with voltage levels of 110 kV and above experience short circuit damage at a rate of 0.5~1 times per 100 units per year.These short circuit faults are responsible for over 50% of all transformer damage accidents.
Gas relay protection is a significant and fundamental protection for power transformers.During a power transformer fault, sudden changes in gas flow speed or insulation oil flow can cause the dry reed of the gas relay to conduct, triggering an alarm signal or tripping command from the signal contact.Gas relay protection should be used as the primary non-electrical protection method for power transformers with a capacity of 800 kVA or higher [3].Gas relay protection can reflect various faults, including a drop in oil level, insulation breakdown, damp heating, or discharge in the iron core or winding of a power transformer.When an alarm or action signal is received from the gas relay, strict inspection and oil sampling tests are necessary to accurately determine the type and nature of the equipment failure.
Only after confirming that the main transformer has no internal faults can it be put back into operation, which can have a significant impact on the reliable operation of the power system and transformer [4].
The gas relay currently in widespread use is a mechanical action device that is vulnerable to malfunctions caused by external faults or interference, potentially leading to shocks and damage to the power system.Under external short circuit impact, accidents of gas relay and pressure relief valve activation, as well as oil spraying, happen repeatedly in transformers.Therefore, it is of utmost importance to conduct research on the impact of external short circuit surges on gas relay operations [5].
The related research methods can be divided into two categories: simulation and actual experiment.Zhou et al. utilized simulation calculations to investigate the impact of external short-circuit impacts on the internal oil flow and gas protection of transformers [6].However, the simulation model was not comprehensive enough, resulting in inaccurate simulation results that inadequately represented the actual physical process.Currently, the actual experiment is the more commonly used research method.Zhou et al. [7] and Huang et al. [8] carried out vibration resistance tests on gas relays and proposed improvement suggestions for gas relay design and acceptance.Nevertheless, these tests were all carried out in a laboratory setting and lacked corresponding measured vibration data for transformers, rendering it impossible to effectively evaluate the anti-vibration performance of gas relays when transformers are subjected to external short-circuit shocks.While analyzing an oil-immersed reactor heavy gas protection malfunction event, Jiang et al. performed seismic capacity testing on gas relays and measured the vibration characteristics of the reactor [9].However, the measured vibration acceleration was during normal operation, not during sudden short circuit impact.
This paper carried out transformer sudden short circuit capability tests and gas relay vibration tests simultaneously.The vibration levels at the gas relay installation location and the vibration characteristics of gas relays during the sudden short-circuit process were analyzed.Based on this analysis, a gas relay detection plan compatible with transformers was established, which can effectively reduce the risk of gas relay malfunction and ensure the safe operation of the power grid.

Fault cases study
On September 11, 2017, heavy gas protection of a certain 220 kV transformer operated, causing the three-side switches to trip.Upon inspection of the gas relay, it was discovered that the opening distance of the heavy gas dry reed contact was too small, making it vulnerable to vibration.It was analyzed that the vibration of the transformer caused the gas relay contacts to falsely operate.
On July 15, 2019, during the first surge test using the 220 kV side switch of a specific 220 kV transformer, the heavy gas protection of the on-load tap changer operated, causing the 220 kV side switch to trip.On-site inspection revealed that the main transformer and on-load tap changer had a normal appearance and all test data was normal, and no fault current was detected during the fault process.The analysis indicated that there were no internal faults in the main transformer and on-load tap changer, and the heavy gas protection operation of the on-load tap changer may have been caused by the false operation of the on-load gas relay.During the surge test of the transformer, the dry reed contact of the gas relay was conducted, causing the heavy gas protection of the on-load tap changer to operate and trip the three-side switch of the transformer.

Experimental model
Combined with the spot check on the capability of withstanding a sudden short circuit, a study on the vibration characteristics of gas relays is carried out.

Sudden short circuit withstand capability test
A sudden short circuit test is specifically designed to assess the comprehensive technical capabilities and manufacturing standards of transformers.The mechanical strength of the transformer body and conductive parts is evaluated using the electromotive force generated by the short circuit current produced by the test system, with the aim of assessing the dynamic stability of the transformer [10,11].
Therefore, the sudden short circuit test is an extremely important and specialized test to ensure the short circuit resistance of transformers.

Vibration test and model
Supplemental fault detection methods, such as noise and vibration analysis, can be used to obtain information about sudden short circuit tests and improve test results [13,14].The test model is shown in Figure 1.
Figure 1.Arrangement of sensors in sudden short circuit test A 220 kV transformer, selected for spot-checking its sudden short circuit withstand capability, was equipped in advance with vibration testing sensors arranged on its wide side, as depicted in the diagram.Vibration sensors 1-4 were arranged uniformly, whereas sensors 5-8 were positioned on the opposite wide side of the transformer.The placement of the vibration sensors avoided the locations of the transformer's reinforcement ribs, and sensor 9 was located around the gas relay.

Vibration test of gas relays
A gas relay vibration testing platform was established in the laboratory to examine the vibration characteristics of gas relay operations.

Basis of the test
The test was carried out in accordance with the provisions for seismic resistance in Article 7.7 of DL/T 540-2013 "Rules for the inspection of Buchholz relay [15]".
The relay was filled with clean transformer oil, and connected to an indicator device at its tripping contact.It was then mounted on a vibration table to undergo a sinusoidal vibration test at a frequency of 4 Hz to 20 Hz with an acceleration of 40 m/s 2 in each of the X, Y, and Z directions for 1 minute.The relay was deemed qualified if the indicator device did not emit any signals.
Note: The axis of the pipe connected to the relay is the X-axis, the Y-axis is perpendicular to the Xaxis in the same horizontal plane, and the Z-axis is perpendicular to the XY plane.

Implementation method in the laboratory
In the laboratory, standard vibration tables and vibration standard kits were utilized to generate vibration.A power amplifier and signal generator were employed to control and display the vibration frequency and acceleration.A digital multimeter was used to indicate the continuity of the tripping contact.
The vibration test platform in the laboratory is shown in Figure 2.

Figure 2. Physical gas relay vibration test platform
The laboratory system for conducting gas relay vibration tests includes a signal generator, vibration test platform, power amplifier, standard sensor(s), digital voltmeter, and supporting fixtures.The specifics of the vibration test system are presented in Table 1.

Analysis of test data
During the sudden short circuit test of the transformer, the maximum vibration acceleration measured by vibration sensors 1 to 8 is shown in Figure 3.As shown in Figure 3, the maximum acceleration of the transformer body vibration does not exceed 2.5 g, whereas the maximum acceleration of the gas relay vibration is close to 3.5 g, indicating greater vibration intensity in the gas relay than in the transformer body.
The vibration test results of the gas relay conducted in the laboratory are shown in Table 2. Table 2 shows that some data sets are less than 3.5 g, suggesting that the gas relay may activate during sudden short circuit faults in the transformer, leading to non-electrical protection misoperation and subsequent tripping of the transformer.

Application case
Up to now, a total of 28 vibration tests of transformer gas relays have been carried out, and 3 internal Reed defects of gas relays have been detected through vibration characteristic tests, which effectively remove abnormal gas relays and ensure the quality of gas relays for power grid access.

Follow-up measures and suggestions
Considering the variety of transformer types (structural form, voltage level, cooling method, etc.), there may be differences in the vibration characteristics of various types of transformers (especially at the gas relay installation points) during sudden short circuit tests.The existing vibration testing methods in the laboratory may not be able to fully simulate the vibration characteristics of transformers during sudden short-circuit circuits.
It is suggested that subsequent measurement and analysis of the vibration characteristics of various types of transformers be carried out, in combination with the short-circuit withstand capability inspection work.This can be used to optimize the laboratory's gas relay vibration testing methods, and improve the vibration resistance and operational reliability of gas relays.

Conclusion
Based on the analysis of the sudden short circuit test and gas relay vibration test, the following conclusions can be drawn regarding the use and maintenance of gas relay: 1.Under the sudden short circuit impact of the power transformer, the gas relay dry reed contacts may conduct and cause protection misoperation, resulting in tripping of the transformer and affecting the safe and stable operation of the power grid.It is recommended to carry out gas relay position vibration tests in conjunction with transformer sudden short-circuit tests to understand the vibration characteristics under various conditions.
2. Vibration tests should be carried out on transformer gas relays for power grid access.Where possible, gas relay anti-vibration performance should be verified under the instantaneous impact, and products whose vibration characteristics do not meet the requirements should be eliminated to ensure the essential safety of the equipment from the source.

Figure 3 .
Figure 3. Vibration acceleration of the transformer body during a sudden short circuit test The maximum vibration acceleration measured by sensor 9 near the gas relay is shown in Figure 4.

Figure 4 .
Figure 4. Vibration acceleration of gas relay during sudden short circuit test

Table 1 .
Components of the gas relay vibration test platform in the laboratory

Table 2 .
Vibration test results of gas relay