The gas relay is installed between the transformer oil tank and the oil storage tank, connected by an oil pipeline with the same diameter as it to form a connected oil circuit. When a serious fault occurs inside the transformer, the oil in the oil tank flows through the pipeline and gas relay to the oil storage tank. If the flow rate reaches the specified value, a trip signal is issued.

1. Flow field of gas relay installed on transformer

When a transformer malfunctions, the oil flow is turbulent, rapidly changing, and unstable. The frequency and amplitude of oil flow surge vary depending on the size of the fault, making it difficult to describe and replicate the instantaneous flow field. So the technical requirements for gas relays are to detect the tripping signal emitted when the oil flow reaches a certain specified value. This requirement has been verified through transformer operation practice to be feasible and is an internationally recognized method. To measure a specified flow velocity value, fluid dynamics require that the flow of oil must be gradual, and the oil flow should be stable and uniform when reaching the specified flow velocity value. To this end, the rapidly changing and unstable flow that actually flows through the gas relay is decomposed into countless continuous and stable uniform flows for measurement. This is the most basic principle for designing a flow rate testing platform.

2. Discussion on Gas Relay Testing Equipment

Gas relay testing equipment can be divided into two categories based on the method of obtaining flow rate values: direct testing method and indirect testing method.

(1) The direct testing method is to install the gas relay in the oil circuit, and the diameter of the oil circuit, the diameter of the gas relay, and the diameter of the pipeline installed on the transformer are equal. That is, the internal and external flow fields of the gas relay on the testing equipment are consistent with the flow field installed on the transformer, so the measured flow rate value is an accurate value that does not require any calibration or correction. All gas relay production enterprises in our country use direct testing equipment for factory testing. Many provincial and municipal power supply companies use direct testing equipment for acceptance and calibration of gas relays. This method is worth promoting vigorously.

(2) The indirect testing method involves not installing the gas relay into the oil circuit, or installing it into an oil circuit with a diameter that is not equal to the diameter of the gas relay. The former has no flow field; The latter has a flow field, but the flow field in the oil pipeline and the flow field at the inlet and outlet of the gas relay are abrupt. The deviation of the measured data is several times greater. So it is necessary to conduct comparative experiments with the direct testing method, calculate the correction coefficients of the two, and make corrections. The corrected flow velocity value is an approximate value.

Based on the above analysis, the testing equipment for gas relays used in transformers must be made using direct testing method. In order to ensure testing accuracy and ease of use, the following conditions should also be met:

(1) The testing equipment consists of three independent testing pipelines with diameters of 25, 50, and 80. Ensure that the diameter of the oil circuit is equal to the diameter of the gas relay, and all three types of gas relays are tested directly.

(2) In addition to having the same diameter, oil pipelines should also be designed with sufficient length of straight pipe sections according to fluid dynamics requirements to ensure uniform oil flow inside the pipeline and ensure testing accuracy.

(3) Design a reasonable speed control valve. Ensure that the oil flow during the speed regulation process is a gradual and stable flow, and that the speed is uniformly regulated throughout the entire process. When stopped, there is no inertia, ensuring stable and uniform oil flow with back pressure.

(4) The accuracy of the turbine flowmeter is level 0.5, and the required flow rate value for measurement is within its range.

(5) The controller used in conjunction with the turbine flowmeter must be certified by the relevant metrology department to ensure accuracy.

(6) The power source should be equipped with a voltage regulator to reduce pulsation and make the oil flow smoother.

(7) The control system adopts human-machine dialogue computer automatic control and automatic printing.