The gas relay is installed between the transformer oil tank and the oil conservator, and is connected by an oil pipeline with the same caliber to form a connected oil circuit. When a serious fault occurs inside the transformer, the oil in the oil tank flows into the oil conservator through the pipeline and gas relay, and a trip signal is issued when the flow rate reaches the specified value.
1. The flow field of a gas relay installed on a transformer
When a transformer fails, the oil flow is a surging, rapidly changing unsteady flow. The size of the fault is different, and the frequency and amplitude of the oil flow surging are also different. The instantaneous flow field is difficult to describe and more difficult to replicate. Therefore, the technical requirements of the gas relay are to detect the trip signal issued when the oil flow reaches a certain value. This requirement is verified by the transformer operation practice and is an internationally accepted method. To measure a certain specified flow rate value, fluid dynamics requires that the flow of oil flow must be slow, and when the specified flow rate value is reached, the oil flow should be a stable and uniform flow. For this reason, the surging unsteady flow that actually flows through the gas relay is broken down into countless continuous stable uniform flows for measurement. This is the most basic principle for designing a flow rate test bench.
2. Discussion on gas relay test equipment
Gas relay test equipment can be divided into two categories according to the method of obtaining the flow rate value: direct test method and indirect test method.
（1） The direct test method is to install the gas relay in the oil circuit, and the pipe diameter of the oil circuit is equal to the diameter of the gas relay and the diameter of the pipeline installed on the transformer, that is, the internal and external flow fields of the gas relay on the test equipment are the same as those installed on the transformer. The flow field on the transformer is consistent, so the measured flow rate value is an accurate value that does not require any calibration and correction. All gas relay manufacturers in my country use the direct test method to make factory tests. Many provincial and municipal power supply companies use the direct test method to accept and set gas relays. This method is worthy of vigorous promotion.
（2） The indirect test method is to not install the gas relay in the oil circuit, or to install the oil circuit pipe diameter and the gas relay diameter not equal to the oil circuit. 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 measured data has a deviation of several times. Therefore, it is necessary to do a comparative test with the direct test method to obtain the two correction coefficients to be corrected. The corrected flow velocity value is an approximate value.
Through the above analysis, the test equipment of the gas relay for transformer must be the test equipment made by the direct test method. In order to ensure test accuracy and ease of use, the following conditions should be met:
（1） The test equipment is composed of three independent test pipes with diameters of 25, 50, and 80. Ensure that the diameter of the oil circuit is equal to the caliber of the gas relay. The gas relays of the three calibers all use the direct test method.
（2） In addition to the same diameter of the oil pipeline, a straight pipe section of sufficient length should be designed according to the requirements of fluid dynamics to ensure that the oil flow in the pipe is uniform and to ensure test accuracy.
（3） Reasonably designed speed control valve. Ensure that the oil flow is a slow and steady flow during the speed adjustment process, and the speed is uniform throughout the entire process. There is no inertia when stopping to ensure that the oil flow is stable and uniform, and there is back pressure.
（4） The accuracy of the turbine flowmeter is 0.5 grade, and the flow rate value required for measurement is within its range.
（5） The controller used in conjunction with the turbine flowmeter must be qualified by the relevant measurement department to ensure accuracy.
（6） The power source should be equipped with a voltage stabilizing device to reduce pulsation and make the oil flow more stable.
（7） The control system adopts man-machine dialogue computer automatic control and automatic printing.