When a symmetrical fault occurs during normal operation, the voltage at the initial stage of the fault may be asymmetric, and a small zero-sequence and negative-sequence voltage may occur in a short time, and the positive-sequence voltage may decrease. When the transformer is air-dropped or over-excited, the terminal voltage will be distorted, gradually changing from asymmetry to symmetry, and the change of the voltage at the other transformer terminal is: the zero-sequence and negative-sequence voltage rises during a fault, and the zero-sequence after the fault is removed. The negative-sequence voltage is reduced to a minimum; the positive-sequence voltage drops when the fault occurs, and the positive-sequence voltage rises to a stable value after the fault is removed.
The asynchronous relationship between the differential current and braking current and the protection start condition, the differential current Id of the transformer, the braking current Ir, and the protection operation constraint conditions in the internal and external faults are abrupt changes in the voltage and current during the fault, and the fault can be determined by equation (2). Start time and act as a start condition for protection. T is the sampling period; X represents voltage and current, φ represents A, B, C three-phase; K is the proportional coefficient, usually takes 1.1-1.3; ΔX is the abrupt quantity, take the half wave effective value; X0 is the fixed threshold value. The use of such a floating threshold startup method can effectively overcome the false start caused by load changes, system oscillations, frequency offsets, and the like.
When a fault occurs outside the area, the differential current is very small during the initial period of protection startup, and then will be affected by the unbalanced factors, and the braking current becomes large after the protection is started, so the braking current and the difference There is a time difference in the increase of flow and it is asynchronous. When a fault occurs in the area, the differential current becomes large after the start of protection, and at the same time, the braking current also becomes large and changes synchronously.
When a fault occurs outside the zone and the CT is saturated, even if the short-circuit current reaches 100 times the rated current, the CT still has a linear transmission time of about 2ms, and the differential current and the braking current increase at the same time and also change asynchronously. In short, when there is a fault in the area, after the start of the protection mutation, if the differential current and the braking current are found to be asynchronous, the ratio differential protection is automatically input.
The fast-ratio differential protection algorithm introduces a method of blocking the terminal voltage 1 according to the size of the terminal voltage sequence component and the difference in the blocking ratio of the change trend. At the same time, a quarter-cycle small vector algorithm or a half-wave recursive algorithm is used to calculate the relevant change amount, and the asynchronous signal is used asynchronously. The method assists in quickly identifying faults in and out of the area. The protection action time can be increased to 1015 ms. Taking the semi-period data window as an example to calculate the relevant voltage value (smaller data window can also be selected), the fast ratio differential protection logic introduces the voltage blocking condition, combined with the asynchronous relationship between the differential current and the braking current, the symmetrical voltage blocking can be obtained. Asynchronous method for fast ratio braking.
The simulation test uses the reduced model of the 220kV prototype system to analyze the protection's operating characteristics, specific parameters, transformer short-circuit impedance 13.5%, high-voltage side turn-to-turn short circuit 1%10% adjustable. Simulate high-, medium-, and low-voltage side faults and high-impedance ground faults, and select a representative test data to verify the reliability of the new algorithm. When the high voltage side of the transformer is dropped, the inrush current and the terminal voltage change, and the positive, negative, and zero sequence voltage variations meet the latching condition, and the protection does not malfunction.
When the transformer is dropped on a slightly severe turn-to-turn fault, the protection operation time does not exceed two cycles, which is related to the set negative-sequence, zero-sequence, and low-value thresholds. The short-circuit turns ratio is still as small as about 3%, but it can still operate fast. However, if the short-circuit turns ratio is small, protection will delay the operation. According to the symmetric equivalent network, the positive sequence, negative sequence, and zero-sequence voltage change in the phase-to-phase and phase-to-phase faults, three-phase faults, and single-phase or two-phase earth faults are very obvious and do not meet the lock-up condition.
The asynchronous relationship between the differential current and braking current and the protection start condition, the differential current Id of the transformer, the braking current Ir, and the protection operation constraint conditions in the internal and external faults are abrupt changes in the voltage and current during the fault, and the fault can be determined by equation (2). Start time and act as a start condition for protection. T is the sampling period; X represents voltage and current, φ represents A, B, C three-phase; K is the proportional coefficient, usually takes 1.1-1.3; ΔX is the abrupt quantity, take the half wave effective value; X0 is the fixed threshold value. The use of such a floating threshold startup method can effectively overcome the false start caused by load changes, system oscillations, frequency offsets, and the like.
When a fault occurs outside the area, the differential current is very small during the initial period of protection startup, and then will be affected by the unbalanced factors, and the braking current becomes large after the protection is started, so the braking current and the difference There is a time difference in the increase of flow and it is asynchronous. When a fault occurs in the area, the differential current becomes large after the start of protection, and at the same time, the braking current also becomes large and changes synchronously.
When a fault occurs outside the zone and the CT is saturated, even if the short-circuit current reaches 100 times the rated current, the CT still has a linear transmission time of about 2ms, and the differential current and the braking current increase at the same time and also change asynchronously. In short, when there is a fault in the area, after the start of the protection mutation, if the differential current and the braking current are found to be asynchronous, the ratio differential protection is automatically input.
The fast-ratio differential protection algorithm introduces a method of blocking the terminal voltage 1 according to the size of the terminal voltage sequence component and the difference in the blocking ratio of the change trend. At the same time, a quarter-cycle small vector algorithm or a half-wave recursive algorithm is used to calculate the relevant change amount, and the asynchronous signal is used asynchronously. The method assists in quickly identifying faults in and out of the area. The protection action time can be increased to 1015 ms. Taking the semi-period data window as an example to calculate the relevant voltage value (smaller data window can also be selected), the fast ratio differential protection logic introduces the voltage blocking condition, combined with the asynchronous relationship between the differential current and the braking current, the symmetrical voltage blocking can be obtained. Asynchronous method for fast ratio braking.
The simulation test uses the reduced model of the 220kV prototype system to analyze the protection's operating characteristics, specific parameters, transformer short-circuit impedance 13.5%, high-voltage side turn-to-turn short circuit 1%10% adjustable. Simulate high-, medium-, and low-voltage side faults and high-impedance ground faults, and select a representative test data to verify the reliability of the new algorithm. When the high voltage side of the transformer is dropped, the inrush current and the terminal voltage change, and the positive, negative, and zero sequence voltage variations meet the latching condition, and the protection does not malfunction.
When the transformer is dropped on a slightly severe turn-to-turn fault, the protection operation time does not exceed two cycles, which is related to the set negative-sequence, zero-sequence, and low-value thresholds. The short-circuit turns ratio is still as small as about 3%, but it can still operate fast. However, if the short-circuit turns ratio is small, protection will delay the operation. According to the symmetric equivalent network, the positive sequence, negative sequence, and zero-sequence voltage change in the phase-to-phase and phase-to-phase faults, three-phase faults, and single-phase or two-phase earth faults are very obvious and do not meet the lock-up condition.
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