A simple method for setting the microcomputer-based differential protection of 35 kV transformers

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With the advancement of technology, integrated automation substations have gradually replaced conventional substations, and microcomputer-based protection systems have also demonstrated their technical advantages. Here, only a brief introduction to the simple setting method for microcomputer-based differential protection in 35 kV transformers is provided. 4 g. `4 p& h3 C% @9 S5 m( ` 1A simple method for setting up ratio-based differential protection3 }, c9 H# J; j3 } 　　To prevent false trips caused by unbalanced differential currents resulting from faults outside the area, transformer differential protection utilizes a ratio-based braking characteristic. Ratio braking differential protection, as the primary protection for transformers, can detect internal phase-to-phase short circuits, single-phase ground short circuits on the high-voltage side, as well as inter-turn short circuits in transformers. 500》) this.style.width=500;"> The ideal braking characteristic curve is a straight line that passes through the origin with a slope equal to the braking coefficient k; the bc line in the figure represents a proportional braking characteristic with a horizontal segment, similar to the ab line in the figure. The operating current at the horizontal line is called the minimum operating current, iop.min. The minimum braking current at which the relay begins to exert a braking effect is known as the knee current, ires.min. Since the break characteristic curve’s broken line does not necessarily pass through the origin, as shown in figure bd. The slope m = (iop – iop.min) / (ires.min) is a constant, whereas the braking coefficient k = iop / irses changes continuously with the braking current; thus, the set ratio braking coefficient kb is actually the slope m of the broken line. / {! c6 S: O( Y/ ~2 K To prevent false operation in the event of faults outside the area, reliance is placed on the braking coefficient k rather than the slope m; therefore, it is necessary to ensure that the k value at each point meets the requirements for selectivity and sensitivity, so that the relay’s braking curve lies above the ideal braking characteristic curve. The braking characteristic curve is determined by three constant values: 1) the proportional braking coefficient kb2. ; l) 　　2) Inflection point current ires.min. 　　3) Minimum operating current iop.min. ) y6 c& y8 f3 X) F# W+ h; e0 H# F 1.1 Setting of the ratio braking coefficient kb: kb=krel(kifi △u △f)9 v7 Y$ `6 B2 H Where krel is the reliability factor, taken as 1.3–1.53; X- U7 _# f; n! z( |' D; @( P ki is the similarity coefficient of the current transformer, taken as 1.0; fi is the maximum relative error of the current transformer, taken as 0.1 when it is 10; △u is the relative error caused by voltage regulation in the transformer, taken as the maximum deviation from the rated value within the voltage regulation range; R1 T9 H5 △f represents the relative error that cannot be completely compensated for due to the transformation ratio of the current transformer. Microcomputer protection software can fully compensate for this, with △f=01 W5 l# @. N; D* Y5 T kb, and the value is generally chosen to be between 0.3 and 0.5. w2 @# h/ }/ d4 o$ X. \) E 1.2 Setting of the inflection point current ires.min 　　It is generally set to the rated current of the transformer. That is: ires.min = ie% C5 }3 \& L( `) p* W7 w 1.3. The setting of the minimum operating current, iop.min, is determined as follows: $ O9 @3 k2 l/ V8 b2 I” z# g% D2 P. It is set so as to meet the requirements regarding braking characteristics, ensuring that the braking coefficient does not change with the braking current; thus, the relationship between the minimum operating current and the current at the inflection point is: iop.min = kbie. With the above settings, both selectivity and sensitivity criteria are met, so there is no need to verify the sensitivity coefficient further. + e. y. }$ i1 p, s, f % i) z, ~0 x2 o3 y2 z; z4 p 2 – Second harmonic blocking function. This function is used to enable differential protection to be blocked by the second harmonics of the differential current, thereby preventing erroneous operation of the differential protection due to inrush currents in the transformer. The criterion for second harmonic braking is as follows: 7 L9 d; Z/ `# z2 S- S& x9 |6 x iop2＞k2*iop7 l/ F- v6 X+ a. Here, iop represents the fundamental component of the differential current; iop2 represents the second harmonic component of the differential current; k2 is the second harmonic braking coefficient, which can take a value of 0.15. - X+ O* x( Z- ~! C8 ^ 3Differential quick-break protection 7 s2 u& t" n' X N: U4 i; a 　To prevent the differential element from failing to operate due to extremely large braking torques generated by an increase in high-order harmonics when the current transformer becomes saturated at high levels of short-circuit current, a differential quick-break element is provided. Differential quick-break protection is not restricted by secondary harmonic current conditions; instead, it avoids inrush currents based on the current value, which results in lower sensitivity. The operating value of the differential quick-break element is generally set at (4-10) times the rated primary current value of the transformer