Main setting principles for differential protection of digital generators

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This post was last edited by chenjinfeng on 2020-3-29 at 17:57. Regarding the longitudinal differential protection of generators, the setting of the operating current for differential quick-break protection is generally determined based on the maximum unbalanced current that can occur due to asynchronous switching of the generator set. Generally, 3 to 4 times the rated current can be used. The setting of the minimum operating current (Ido) for ratio differential protection is determined as the minimum value required for the generator differential protection to operate. It should be set such that it can handle the maximum unbalanced current (Iunb·o) that occurs under normal rated load conditions on the generator; that is, Ido = Kk·Iunb·o, or Ido = Kk×2×0.03If2n. Here, Kk represents the reliability factor, with a value of 1.5 ； Iunb·o — The measured unbalanced current in the differential protection under the rated load condition of the generator ； If2n—secondary rated current of the generator. Generally, Ido can be set to (0.15～0.3 In), with 0.2 In being the usual value. If the measured value of Iunb·o is high, the cause of this increase should be identified as soon as possible and eliminated, in order to prevent excessive setting of Ido from masking defects or potential problems in the primary and secondary equipment. When there is an internal short circuit in the generator, especially one that occurs through a transition resistor near the neutral point, the three-phase currents at the generator terminals or on the neutral side may not be high. To ensure sensitivity in the event of an internal short circuit, the minimum operating current Ido should not be increased unnecessarily. The knee-point current value (Iro) is set such that when the stator current is equal to or less than the rated current, the differential protection does not need to have a braking characteristic; therefore, Iro can be set as: Iro = (0.8–1.0)If2n. The proportional braking coefficient (K) for the generator’s differential protection is determined using the following formula: K = Kk·Kap·Kcc·Ker, where Kk is the reliability factor, taken as 1.5, and Kap is the coefficient for aperiodic components, taken as 2.0 ； Kcc—Current transformer similarity coefficient, taken as 1.0 ； Ker—current transformer ratio error, taken as 0.1. In practical engineering applications, K=0.3 is usually adopted to ensure safety and reliability. Sensitivity verification: For a differential protection system with a proportional braking characteristic set in accordance with the principles mentioned above, when there is a metallic short circuit between two phases at the generator terminal, the sensitivity of this differential protection will certainly meet the required standards, so no sensitivity verification is necessary. Transverse differential protection: The setting value of the operating current for this type of protection (Ihczd) is determined in such a way as to allow it to withstand the maximum unbalanced current resulting from external short circuits. Generally, Ihczd can be taken as (0.2～0.3)If1n/na, where If1n represents the rated primary current of the generator ； na—Ratio of the zero-sequence current transformer for generator transverse difference. High-sensitivity differential protection: The setting value of the operating current for this type of protection (Ihczd) is determined in such a way as to allow it to withstand the maximum imbalance current in the neutral connection during normal operation of the generator. Generally, Ihczd can be set as: Ihczd = (0.05～0.1)If1n/na, where If1n represents the rated primary current of the generator ； na—Ratio of the zero-sequence current transformer for generator transverse difference. As the operating value of this protection operating current, it should correspond to the fundamental and third harmonic components of the zero-sequence unbalanced current generated on the secondary side of the cross-current transformer due to avoiding the maximum external short-circuit current as well as rotor eccentricity in the event of loss of magnetism or synchronization. The operating value of the sensitive differential current setting can be determined using the following formula: Ihczd = Kk·Kap, where Kk is the reliability factor, taken as 1.5 ； Kap—coefficient of the aperiodic component, ranging from 1.5 to 2.0 ； Iunb·1·max—the fundamental zero-sequence unbalanced current at the secondary side of the cross-difference TA under maximum external short-circuit current ； Iunb·3·max—the third-harmonic unbalanced current at the secondary side of the cross-difference TA under maximum external short-circuit current ； K3—third harmonic filtering ratio, taken as 100. The knee current and the setting knee current Iro for the high-sensitivity mode difference protection are generally taken as the secondary rated current If2n of the generator. The phase current braking coefficient is generally set between 0.2 and 0.5. The operating time is set to occur after a single-point ground fault in the generator excitation circuit; to prevent false operation of the differential protection in the event of a transient two-point ground fault in the excitation circuit, the protection switching time is set to 0.5–1 second.