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This post was last edited by chenjinfeng on 2020-4-6 at 14:28. Main principles for setting the parameters of the grounding protection device in digital generators. Stator grounding protection using fundamental zero-sequence voltage: The operating voltage (Uodz) should be set such that it can withstand the maximum unbalanced voltage (Uunb·max) in the open delta winding of the single-phase voltage transformer at the neutral point or of the three-phase voltage transformers at the machine end during normal operation. The calculation formula is as follows: Uodz = Kk·Uunb·max, where Kk is a reliability factor, taken as 1.2 to 1.3； Uunb·max—measured unbalanced voltage. In practical engineering applications, it is usually set as: Uodz = (0.1～0.2)Ue. When a zero-sequence voltage blocking criterion is applied to the high-voltage side of the main transformer, the operating voltage (Uodz) and the blocking voltage (Uhbs) are set according to the following formulas: Uodz = (0.05～0.1)Ue, and Uhbs = 15～20V. The setting of the action time should be coordinated with the system grounding protection. Stator ground protection using the third harmonic voltage ratio: The values of the third harmonic voltage ratio (K31, K32) must be determined based on actual measurements of the third harmonic voltage ratios before and after grid connection. The method for setting these values is as follows: Preliminary setting: α = Kk, where Kk is a reliability factor ranging from 1.3 to 1.5 ； nTVO—Turn ratio of the open-core delta zero-sequence voltage transformer at the machine side ； nTVN—Ratio of the neutral point zero-sequence voltage transformer. If the maximum third harmonic voltage ratio of the generator before grid connection, as measured in practice, is α1, then the preset value for this ratio before grid connection is: K31 = (1.3–1.5)α1 ； If the maximum third harmonic voltage ratio of the generator after connection to the grid is measured as α2, then the fixed value for this ratio after grid connection is: K32 = (1.3～1.5)α2 ； The setting of the action time should be coordinated with the system grounding protection. Stator ground protection with self-adjusting third harmonic voltage: The value of the self-adjusting ratio K for the third harmonics should be set such that, during normal operation of the generator, the braking value K︱U3N︱ remains always greater than the triggering value; generally, K is set between 0.2 and 0.3. The actuation time must avoid the delay of the backup protection for faults outside the zone. Zero-sequence current stator ground protection: The operating value of zero-sequence current (Iodz) is set to accommodate the maximum unbalanced current that occurs in the event of a single-phase ground fault outside the generator accompanied by a two-phase short circuit. The calculation formula for this value is Iodz = (Kk·3Icf + K′KIbph), where Kf is the return coefficient, taken as 0.9 ； Kk—reliability coefficient, taken as 2–3 (a smaller value is used when the operation time is longer). K′K—reliability coefficient, taken as 1.5 ； 3Icf—Capacitive current of the protected generator ； Ibph—the maximum unbalanced current generated on the secondary side of the zero-sequence current transformer in the event of an external short circuit. The operating time of the zero-sequence current stator ground protection is generally set between 0.5 and 1 second. Generator rotor ground protection: Single-point rotor ground protection. This protection operates to send a signal when a single-point ground occurs on the rotor. The setting value of the ground resistance (Rgzd) varies depending on the cooling method used in turbo-generators and hydro-generators. Typically, for hydro-generators as well as air-cooled and hydrogen-cooled turbo-generators, this value is set at 20 KΩ. For excitation windings cooled directly by water, it is generally set at 2.5 KΩ. The action time can usually be set between 5s and 10s. Rotor two-point grounding protection; the setting value for the displacement detection in this type of protection can generally be set to 20%. The actuation time is set based on avoiding momentarily occurring two-point ground faults, and is generally between 0.5 s and 1 s.