1. Strictly check the current transformers used. The two sets of current transformers at the tail end and the outlet circuit breaker used for generator differential protection should be selected from one manufacturer, and the model and performance should be identical. The three-phase ratio must be the same, and try to use the D-class core current transformer. When the maximum short-circuit current of the external short-circuit fault passes, the current transformer should be able to meet the requirements of the 10% error curve, thereby avoiding the malfunction caused by the unbalanced current generated by the difference in the parameter performance of the current transformer itself.
2. The protection wiring must be correct. If any one of the phases is reversed, an unbalanced current will be generated and the protection will malfunction. Because the differential protection is the main protection reflecting the phase-to-phase short-circuit fault of the stator winding of the generator, the secondary side of the current transformer should be connected according to the circulating current method. The terminals of the same polarity of the current transformers at both ends should face in the same direction, that is, two currents. The secondary side of the transformer is connected with different polarity, and the current coil of the differential relay is connected in parallel between the lines of the two current transformers. At the same time, in order to reduce the influence of the maximum unbalanced current in the transient process, the differential protection (relay) circuit should be connected to the differential circuit via the intermediate speed saturated converter, and the non-periodic component is not easily transmitted to the fast saturation converter. On the secondary side, the intermediate-speed saturable converter can successfully escape the influence of the non-periodic component of the maximum unbalanced current that occurs at the moment of the external initial short-circuit.
3. When calculating and checking the protection value, it should be determined according to different categories of generator differential protection. It adopts conventional electromagnetic type and semiconductor type protection, and mainly has differential braking type and differential protection with DC magnetic assisting characteristics. In the calculation of the ratio braking type differential protection setting, according to the characteristics of the ratio braking type differential protection action, two fixed values, that is, the minimum operating current and the braking coefficient are calculated, and the minimum operating current cannot be smaller than the inherent operating current of the relay. The procedure requires that the generator differential protection braking current with ratio braking be Idz=(0.1~0.3)IN, generally 0.2IN, and the braking coefficient is recommended to be 0.3~0.4. When calculating the differential protection value of DC-assisted magnetic characteristics, the operating current should be set according to the maximum unbalanced current when the external fault is avoided and the secondary circuit disconnection of the current transformer. At the same time, the current loop disconnection signal should be set. .
4. Microcomputer-based protection, mainly with sampling instantaneous value and fundamental phasor longitudinal differential protection. Microcomputer-type generator differential differential protection should be used to prevent the influence of interference. The protection device should have stable and reliable grounding. The main reason is that the device casing should be grounded. If the manufacturer has no special requirements, the grounding resistance should not be greater than 1 Ω. The second is the digital, analog, power, shielding and other units inside the device. In order to prevent the interference of electric and magnetic fields, a shielded secondary control cable must be used, and the device casing uses ferrous materials. In order to prevent external surges from affecting the operation of the microcomputer, it is necessary to ensure that there is no power connection between any point of the terminal block and the microcomputer. The power supply should be used with caution. Generally, the Switching Power Supply after the inverter should be used. The battery is DC-inverted into a high-frequency voltage and then isolated by a high-frequency transformer.
5. To ensure the sensitivity of the differential protection action, ensure that the secondary circuit of the current transformer does not malfunction when it is disconnected. The balance coil is introduced in the differential protection wiring, that is, the differential coil is connected to the corresponding phase line in the differential circuit, and the three balance coils are connected in series in the neutral line loop and connected to the differential coil in reverse polarity. Thus, when the connecting wire of any of the differential circuits is broken, a differential coil and three balancing coils on the broken phase have current passing through, because the differential coil of the broken phase and its balancing coil are reverse polarity. When connected, the generated magnetic fluxes cancel each other out, so the differential protection does not malfunction due to disconnection.
6. The preventive test must be carried out for the switching and replacement of the protection device, and the secondary circuit of the generator set should be inspected. During the test, test the DC resistance of the secondary circuit of the differential protection, check whether the wiring is secure, and whether the circuit wiring is correct. After installation and maintenance, before the operation is put into operation, the polarity of the current transformer should be checked, that is, the primary side current flows from the terminal L1, and the secondary side current flows out from the same polarity terminal K1, using the direct current method and the alternating current. The direct measurement method of the method and the instrument checks the polarity of the current transformer.
7. After the genset differential protection action trips, the test power transmission should not be carried out easily. All equipment within the protection scope should be carefully checked. First check from the outside, check whether there is obvious short circuit outside the equipment, check the wiring of the secondary circuit, the polarity is not obvious wiring error, and then use the testing equipment for preventive test to check whether there is a short circuit between the generators. There is no short circuit in the lead-out part of the generator.
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