Safe processing method for active voltage reduction of ground fault phase of non-effective ground system

Abstract

The proposed invention discloses a safe processing method for active voltage reduction of a ground fault phase of a non-effective ground system, for use in safe processing of a ground fault of a neutral point non-effective ground generator or distribution network. Non-effective ground system side windings of a transformer are equipped with a plurality of shunting taps. When a single-phase ground fault occurs, the shunting tap of the fault phase winding is selected to be short-circuited to ground directly or through an impedance to reduce the fault phase voltage to ensure that the fault point voltage is less than the continuous burning voltage of a ground arc to meet a long-term non-stop safe operation requirements. The proposed method can eliminate the instantaneous single-phase ground fault, suppress the permanent single-phase ground fault current, and limit the rising amplitude of non-fault phase voltage and the risk of non-fault phase insulation breakdown.

Claims

1. A safe processing method for active voltage reduction of a ground fault phase of a non-effective ground system, for use in safe processing of a ground fault of a neutral point non-effective ground generator or distribution network, wherein non-effective ground system side windings of a transformer are provided with a plurality of shunting taps, serial numbers of shunting taps of each phase are defined to sequentially increase from a neutral point to an output, the shunting tap X is short-circuited to a ground to force an output voltage of the phase to be Ux=E.Math.(NN.sub.x)/N, and the larger serial number of the shunting tap short-circuited to the ground is, the lower the output voltage of the corresponding fault phase is; when a single-phase ground fault occurs, a transformer shunting tap is selected according to a target value U.sub.2 of a voltage reduction operation of the ground fault phase, and selected the shunting tap with a smallest serial number, selected according to the fact that the number of turns of coils from the neutral point to the shunting tap is greater than NN.Math.U.sub.2/E, is short-circuited to the ground to implement safe operation processing of active voltage reduction, wherein E is a power phase voltage, N is a total number of turns of coils of each phase of winding, N.sub.x is a number of turns of coils from the shunting tap X to the neutral point in a fault phase winding, the target value U.sub.2 of the voltage reduction operation of the ground fault phase is (0, U.sub.1), and U.sub.1 is a fault phase voltage before the shunting tap is short-circuited to the ground.

2. The safe processing method for active voltage reduction of the ground fault phase of the non-effective ground system according to claim 1, wherein during voltage reduction operation of the distribution network, a zero sequence current of a ground fault line is measured; if the zero sequence current is greater than a threshold, the shunting tap is sequentially increased and changed to short-circuit to the ground, so that the fault phase voltage is further reduced to suppress the fault current until the zero sequence current of the ground fault line is smaller than or equal to the threshold, and safe operation of active voltage reduction of the ground fault phase is achieved.

3. The safe processing method for active voltage reduction of the ground fault phase of the non-effective ground system according to claim 2, wherein a zero sequence current threshold is selected based on the fault current allowed for long-time safe operation of a line with a single-phase ground fault and is [1A, 30A], or selected based on the suppression rate of the ground fault current and is [0.001 I.sub.0, I.sub.0], where I.sub.0 is a zero sequence current of the fault line before the shunting tap is short-circuited to the ground.

4. The safe processing method for active voltage reduction of the ground fault phase of the non-effective ground system according to claim 1, wherein during the voltage reduction operation, the exit current of the non-effective ground system side windings is measured and calculated, and the shunting tap is sequentially increased and changed to short-circuit to the ground to establish formula ={dot over (U)}.sub.0.Math.Y.sub.0, so that an arc of the fault point is suppressed, where Y.sub.0 is a zero sequence admittance to the ground when the non-effective ground system runs normally.

5. The safe processing method for active voltage reduction of the ground fault phase of the non-effective ground system according to claim 1, wherein during the voltage reduction operation, a damping rate $d=\frac{g}{C}=\frac{U_{0}g}{U_{0}C}=\frac{I_{0R}}{I_{0C}}=\frac{P_0}{Q_0}=\cot {}_0$ of the non-effective ground system or the ground fault line is measured and calculated; if the damping rate d is greater than a threshold, the shunting tap is sequentially increased and changed to short-circuit to the ground, so that the fault phase voltage is further reduced to suppress the fault arc until d is smaller than or equal to the threshold, that is, fault arc blowout is determined, and safe operation of active voltage reduction of the ground fault phase is achieved; where g is a three-phase conductance to the ground, is an angular frequency of the system, C is a three-phase capacitance to the ground, and U.sub.0 is a zero sequence voltage; I.sub.0R is a zero sequence active current, and I.sub.0C is a zero sequence capacitance current; P.sub.0 is a zero sequence active power, Q.sub.0 is a zero sequence reactive power, and .sub.0 is a zero sequence admittance angle.

6. The safe processing method for active voltage reduction of the ground fault phase of the non-effective ground system according to claim 1, wherein when the shunting tap is short-circuited to the ground, in order to prevent excessive inrush current, the shunting tap is firstly short-circuited to the ground through an impedance Z; if a short-circuit current is smaller than a short-circuit current threshold, short-circuiting the impedance Z, so that the shunting tap X is directly short-circuited to the ground; otherwise, a fault phase selection error is determined, and the impedance Z is disconnected.

7. The safe processing method for active voltage reduction of the ground fault phase of the non-effective ground system according to claim 6, wherein a value of the impedance Z is [10, 500] ohm; a threshold of the short-circuit current is K.sub.1U.sub.0/Z.sub.0, where U.sub.0 is measured zero sequence voltage, Z.sub.0 is a zero sequence impedance of the non-effective ground system during normal operation, K.sub.1 is a safety factor and is [1, 3].

8. The safe processing method for active voltage reduction of the ground fault phase of the non-effective ground system according to claim 1, wherein a protection device is arranged between the shunting tap X and the ground to prevent a high current flowing through a short-circuited loop to damage equipment.

9. The safe processing method for active voltage reduction of the ground fault phase of the non-effective ground system according to claim 1, wherein the transformer is a Z-type ground transformer or a Y/ wiring transformer or a Y/Y/ wiring transformer.

10. The safe processing method for active voltage reduction of the ground fault phase of the non-effective ground system according to claim 1, wherein a number of shunting taps of each phase of winding at the non-effective ground system side is set in a range of 1-30.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a principle diagram of a safe processing method for active voltage reduction of a ground fault phase of a non-effective ground distribution network using a Y/ wiring transformer.

(2) FIG. 2 is a zero sequence equivalent circuit diagram of a non-effective ground system when a ground fault occurs.

(3) FIG. 3 is a phasor diagram of a voltage reduction are suppression operation range of a ground fault phase of the non-effective ground system.

(4) FIG. 4 is a measurement principle diagram of damping rate of a non-effective system or a line.

(5) FIG. 5 is a principle diagram of a safe processing method for active voltage reduction of a ground fault phase of a non-effective ground distribution network using a Z-type ground transformer.

(6) FIG. 6 is a principle diagram of a safe processing method for active voltage reduction of a ground fault phase of a non-effective ground generator.

DETAILED DESCRIPTION

(7) The following further describes and interprets the present invention with reference to the accompanying drawings.

(8) As shown in FIG. 1, in a non-effective ground distribution network, E.sub.A, E.sub.B and E.sub.C are three phases of power electromotive forces of a system, C.sub.0 is system capacitance to ground, r.sub.0 is leakage resistance to ground, A.sub.s, B.sub.s and C.sub.s are non-effective ground system side windings of a Y/ wiring transformer, S is a switch, P is a protection device (an overcurrent protection device or a fuse), outlet lines at respective one ends of the non-effective ground system side windings of the transformer are directly connected with three phases A, B and C of a non-effective ground system, and the non-effective ground system side windings of the transformer are star-connected to lead out a neutral point N and then grounded through an impedance Z.sub.1; a.sub.s, b.sub.s and c.sub.s are low-voltage side windings of the transformer, and the low-voltage side windings are delta-connected; the non-effective ground system side windings of the transformer are provided with a plurality of shunting taps (the plurality of shunting taps refers to totally three or more shunting taps of three phases of windings A, B and C), the number of shunting taps of each phase of winding is 1 to 30, the serial numbers of shunting taps of each phase are defined to sequentially increase from the neutral point to the terminal, and any shunting tap X is short-circuited to ground to force the output voltage of the phase to be U.sub.X=E*(NN.sub.X)/N, where E is a power phase voltage, N is a total number of turns of coils of each phase of winding, and N.sub.X is a number of turns of coils from the shunting tap X to the neutral point in a fault phase winding. At the same time, in order to prevent high current from flowing through the short-circuited loop to damage equipment, a protection device P is installed between the shunting tap X and the ground. When a single-phase ground fault occurs, the ground fault resistance is R.sub.f, and a transformer shunting tap is selected according to a target value U.sub.2 of voltage reduction operation of the ground fault phase. The shunting tap with the smallest serial number, selected according to the fact that the number of turns of coils from the neutral point to the shunting tap is greater than NN.Math.U.sub.2/E, is short-circuited to ground to implement safe operation processing of active voltage reduction; where the target value U.sub.2 of voltage reduction operation of the ground fault phase is (0, U.sub.1), and U.sub.1 is a fault phase voltage before the shunting tap is short-circuited to ground.

(9) A zero sequence equivalent circuit in the non-effective ground system corresponding to FIG. 1, i.e., a zero sequence equivalent circuit of the non-effective ground system when a ground fault occurs, is as shown in FIG. 2. According to the Kirchhoff current equation, the output current of the non-effective ground system side windings of the transformer is:

(10) $\begin{array}{cc}\stackrel{.}{I}=j3{\stackrel{.}{U}}_0{C}_0+\frac{3{\stackrel{.}{U}}_0}{r_0}+\frac{{\stackrel{.}{U}}_C}{R_f}+\frac{{\stackrel{.}{U}}_0}{Z_1}={\stackrel{.}{U}}_0.Math.Y_0+{\stackrel{.}{U}}_0{Y}_f.& \left(1\right)\end{array}$

(11) In equation (1), the zero sequence admittance to ground of the distribution network is

(12) $.Math.Y_0=\frac{1}{Z_1}+j3C_0+\frac{3}{r_0}=Y+jC+g,$
the grounding admittance of the neutral point is

(13) $Y=\frac{1}{Z_1},$
the three-phase conductance to ground is

(14) $g=\frac{3}{r_0},$
the three-phase capacitance to ground is C=3C.sub.0, the fault conductance to ground is

(15) $Y_f=\frac{1}{R_f},$
and {dot over (U)}.sub.0 is a zero sequence voltage.

(16) Considering the zero sequence voltage effect caused by asymmetry of three-phase ground parameters under normal operation of the non-effective ground system, the zero sequence voltage U.sub.0 in equation (1) is replaced by a zero sequence voltage variation {dot over (U)}.sub.0; and considering the grounding admittance Y.sub.f=0 of the fault point after fault arc suppression, equation (1) may be simplified as:
={dot over (U)}.sub.0.Math.Y.sub.0=({dot over (U)}.sub.03{dot over (U)}.sub.01)Y.sub.0=({dot over (U)}.sub.3{dot over (U)}.sub.1).Math.Y.sub.0.(2)

(17) Thus, during voltage reduction operation, the output current of the non-effective ground system side windings is measured and calculated, and the shunting tap is sequentially increased and changed to short-circuit to ground to establish formula =U.sub.0.Math.Y.sub.0, so that the are of the fault point is suppressed, where Y.sub.0 is a zero sequence admittance to ground when the non-effective ground system runs normally.

(18) In the present embodiment, when a shunting tap is short-circuited to ground, in order to prevent excessive inrush current, the shunting tap is firstly short-circuited to ground through the impedance Z. If the short-circuit current is smaller than a short-circuit current threshold, the impedance is short-circuited, so that the shunting tap X is directly short-circuited to ground. Otherwise, a fault phase selection error is determined, and the impedance is disconnected.

(19) The following further discusses a fault phase voltage reduction operation range of fault are suppression. As shown in FIG. 3, when the system is in normal operation, the voltage of the neutral point is zero, the A phase voltage vector is {right arrow over (OA)}, the B phase voltage vector is {right arrow over (OB)}, and the C phase voltage vector is {right arrow over (OC)}; taking the ground fault of the C phase as an example, if the maximum operating voltage amplitude of the fault phase ensuring fault phase are suppression is CC, the condition of fault phase are suppression is: the zero potential point is within a circle centering on C and having a radius of CC; in addition, in order to prevent insulation breakdown caused by excessive voltage of a non-fault phase, the voltage of the non-fault phase is required to be smaller than a line voltage, that is, the zero potential point should be within a circle centering on point A and having a radium of AC, and a circle centering on point B and having a radium of BC. Thus, in order to ensure long-time safe operation of the non-effective ground system after voltage reduction of the fault phase, the zero potential point after voltage reduction of the fault phase is within an intersection of the three circles.

(20) The following further discusses a method of judging fault are suppression by measuring a damping rate. As shown in FIG. 4, during voltage reduction are suppression, the damping rate of the system is calculated by measuring the zero sequence current .sub.0s and zero sequence voltage of the system, or the damping rate of the fault line m is calculated by measuring the zero sequence current .sub.0m and zero sequence voltage of the fault line m. The calculation formula of the damping rate of the non-effective ground system or the damping rate of the line is:

(21) $d=\frac{g}{C}=\frac{U_{0}g}{U_{0}C}=\frac{I_{0R}}{I_{0C}}=\frac{P_0}{Q_0}=\cot {}_0,$
and the threshold of the damping rate d is set to be K.sub.3 times the damping rate of the system or the line in normal operation; the coefficient K.sub.3 is in a range of [1,5]; if the damping rate d is greater than the threshold, the shunting tap is sequentially increased and changed to short-circuit to ground, so that the fault phase voltage is further reduced to suppress the fault arc until d is smaller than or equal to the threshold, that is, fault arc blowout is determined, and safe operation of active voltage reduction of the ground fault phase is achieved; where

(22) $g=\frac{3}{r_0}$
is three-phase
conductance to ground, is angular frequency of the system, C=3C.sub.0 is three-phase capacitance to ground, and U.sub.0 is zero sequence voltage; I.sub.0R is zero sequence active current, and I.sub.0C is zero sequence capacitance current; P.sub.0 is zero sequence active power, Q.sub.0 is zero sequence reactive power, and .sub.0 is zero sequence admittance angle.

(23) As shown in FIG. 5, the Y/ wiring transformer in the present embodiment may be replaced by a Z-type ground transformer. Similarly, the Y/ wiring transformer in the present embodiment may also be replaced by a Y/Y/ wiring transformer.

(24) The above describes the technical principle of the present invention applied to a non-effective ground distribution network in detail. The technical principle is also applicable to the case where the present invention is applied to a non-effective ground generator. The following further describes the application of the present invention to the non-effective ground distribution network and generator:

(25) In the first case, as shown in FIG. 1, in the non-effective ground distribution network, E.sub.A=E.sub.B=E.sub.C=10/{square root over (3)}kV, the leakage resistance to ground of line r.sub.0 is 4.7 k, the capacitance to ground of line C.sub.0 is 8.36 uF, P is a protection device (an overcurrent protection device or a fuse), the target value U.sub.2 of voltage reduction operation of the ground fault phase is set to 2.4 kV, the zero sequence current threshold of the ground fault line is 10 A, the ground impedance Z.sub.1 of the neutral point N is j121, A.sub.s, B.sub.s and C.sub.s are non-effective ground system side windings of the Y/ wiring transformer, the outgoing lines of A.sub.s, B.sub.s and C.sub.s are respectively connected to three phases of buses A, B and C, the total numbers (N) of turns of coils of the windings A.sub.s, B.sub.s and C.sub.s are respectively 150, totally 15 shunting taps are arranged in the windings A.sub.s, B.sub.s and C.sub.s, that is, the windings A.sub.s, B.sub.s and C.sub.s are respectively provided with 5 shunting taps, and the serial numbers of shunting taps of each phase are defined to sequentially increase from the neutral point to the terminal, respectively shunting tap 1, shunting tap 2, shunting tap 3, shunting tap 4, and shunting tap 5; the number of turns of coils from the shunting tap 1 to the neutral point is 30, the number of turns of coils from the shunting tap 2 to the neutral point is 60, the number of turns of coils from the shunting tap 3 to the neutral point is 90, the number of turns of coils from the shunting tap 4 to the neutral point is 120, and the number of turns of coils from the shunting tap 5 to the neutral point is 150. Before a single-phase ground fault occurs in phase C and the shunting taps are not short-circuited, it is detected that the voltage U.sub.1 of the fault phase is 2.6 kV and the ground fault resistance R.sub.f is 1 k. At this time, the voltage U.sub.1 of the fault phase C is subjected to voltage reduction for safe processing, the shunting tap 3 with the smallest serial number, selected according to the fact that the number of turns of coils from the neutral point to the shunting tap is greater than NN.Math.U.sub.2/E=88.5, is short-circuited to ground. The number N.sub.3 of turns of coils from the shunting tap 3 to the neutral point is 90, and the fault phase voltage is then reduced to U.sub.3=E.sub.C.Math.(NN.sub.3)/N=2.3 kV, which satisfies the voltage operation range [0, 2.60 kV] of the fault phase. At the moment, the non-fault phase voltage is 7.2 kV, which is smaller than the line voltage 10 kV. Thus, arc suppression of the ground fault phase is realized. Meanwhile, the non-fault phase voltage also does not rise to the line voltage, so that safe operation processing of active voltage reduction is achieved.

(26) During voltage reduction safe processing, the zero sequence current of the ground fault line is measured. If it is greater than the threshold 10 A, the shunting tap is sequentially increased and changed to short-circuit to ground, so that the fault phase voltage is further reduced to suppress the fault current until the zero sequence current of the ground fault line is smaller than or equal to the threshold 10 A, and safe processing of active voltage reduction of the ground fault phase is achieved.

(27) In order to prevent excessive inrush current, the shunting tap is firstly short-circuited to ground by the impedance Z. If the short-circuit current is smaller than the short-circuit current threshold, the impedance is short-circuited, so that the shunting tap X is directly short-circuited to ground. Otherwise, a fault phase selection error is determined, and the impedance is disconnected. The value of the impedance Z is 10 ohm.

(28) In the second case, as shown in FIG. 6, in the non-effective ground generator, E.sub.A=E.sub.B=E.sub.C=20/{square root over (3)}kV, the ground leakage resistance r.sub.0 of the generator stator is 20 k, the ground capacitance C.sub.0 of the generator stator is 1.81 uF, P is a protection device (an overcurrent protection device or a fuse), the target value U.sub.2 of voltage reduction operation of the ground fault phase is set to 2.6 kV, the threshold of the damping rate d is K.sub.3 times the damping rate of the system or the line in normal operation, the coefficient K.sub.3 is 3, the damping rate in normal operation is 8.8%, the ground impedance Z.sub.2 of the neutral point N is j600, A.sub.s, B.sub.s and C.sub.s are non-effective ground system side windings of the Y/ wiring transformer, the outgoing lines of A.sub.s, B.sub.s and C.sub.s are respectively connected to three phases of buses A, B and C, the total numbers (N) of turns of coils of the windings A.sub.s, B.sub.s and C.sub.s are respectively 150, totally 15 shunting taps are arranged in the windings A.sub.s, B.sub.s and C.sub.s, that is, the windings A.sub.s, B.sub.s and C.sub.s are respectively provided with 5 shunting taps, and the serial numbers of shunting taps of each phase are defined to sequentially increase from the neutral point to the exit, respectively shunting tap 1, shunting tap 2, shunting tap 3, shunting tap 4, and shunting tap 5; the number of turns of coils from the shunting tap 1 to the neutral point is 30, the number of turns of coils from the shunting tap 2 to the neutral point is 60, the number of turns of coils from the shunting tap 3 to the neutral point is 90, the number of turns of coils from the shunting tap 4 to the neutral point is 120, and the number of turns of coils from the shunting tap 5 to the neutral point is 150.

(29) Before a single-phase ground fault occurs in phase C and the shunting taps are not short-circuited, it is detected that the voltage U.sub.1 of the fault phase is 2.76 kV and the ground fault resistance R.sub.f is 2 k. At this time, the voltage U.sub.1 of the fault phase C is reduced for safe processing, the shunting tap 4 with the smallest serial number, selected according to the fact that the number of turns of coils from the neutral point to the shunting tap is greater than N-N.Math.U.sub.2/E=116.2, is short-circuited to ground. The number N.sub.4 of turns of coils from the shunting tap 4 to the neutral point is 120, and the fault phase voltage is then reduced to U.sub.4=E.sub.C.Math.(NN.sub.4)/N=2.31 kV, which satisfies the voltage operation range [0, 2.76 kV] of the fault phase. At the moment, the non-fault phase voltage is 14.42 kV, which is smaller than the line voltage 20 kV. Thus, arc suppression of the ground fault phase is realized. Meanwhile, the non-fault phase voltage also does not rise to the line voltage, so that safe operation processing of active voltage reduction is achieved.

(30) During voltage reduction safe processing, the damping rate of the non-effective ground generator is measured. If the damping rate d is greater than the threshold 38.8%=26.4%, the shunting tap is sequentially increased and changed to short-circuit to ground, so that the fault phase voltage is further reduced to suppress the fault arc until d is smaller than or equal to the threshold 38.8%=26.4%, that is, fault arc blowout is determined, and safe operation of active voltage reduction of the ground fault phase is achieved

(31) In order to prevent excessive inrush current, the shunting tap is firstly short-circuited to ground by the impedance Z. If the short-circuit current is smaller than the short-circuit current threshold, short-circuiting the impedance, so that the shunting tap X is directly short-circuited to ground. Otherwise, a fault phase selection error is determined, and the impedance is disconnected. The value of the impedance Z is 10 ohm.