Safe operation method for voltage reduction arc suppression of ground fault phase of non-effective ground system

Abstract

The present invention discloses a safe operation method for voltage reduction arc suppression of a ground fault phase of a non-effective ground system, for use in ground fault safety operation of a neutral point non-effective ground generator or distribution network. When a single-phase ground fault occurs, an external voltage source is applied at a non-effective ground system side between a bus and the ground, or between a line and the ground, or between a neutral point and the ground, or between a shunting tap of a non-effective ground system side winding of a transformer and the ground, to reduce the fault phase voltage to be lower than the continuous burning voltage of a ground arc, thereby meeting the requirements of long-term non-stop safe operation. The operation means and control method effectively prevent power outages, and improve the reliability and security of power supply.

Claims

1. A safe operation method for voltage reduction arc suppression of a ground fault in a phase of a non-effective ground system, for use in ground fault safe operation of a neutral point non-effective ground generator or distribution network, wherein when a single-phase ground fault occurs, an external voltage source is applied at a non-effective ground system side between a bus and a ground, or between a line and the ground, or between a neutral point and the ground, or between a shunting tap of a non-effective ground system side winding of a transformer and the ground, and a voltage output by the external voltage source is {dot over (U)}={dot over (U)}.sub.1+Δ{dot over (U)}.sub.0, so that a fault phase voltage is reduced to achieve voltage arc suppression and an active voltage reduction operation of the ground fault; where {dot over (U)}.sub.1 is a normal voltage of an access point when a normal grid voltage source is not connected, a variation of zero sequence voltage Δ{dot over (U)}.sub.0 is calculated from formula Δ{dot over (U)}.sub.0={dot over (U)}.sub.03−{dot over (U)}.sub.01 or Δ{dot over (U)}.sub.0={dot over (U)}.sub.φ1−Ė.sub.φ, {dot over (U)}.sub.03 is a zero sequence voltage after the active voltage reduction operation, {dot over (U)}.sub.01 is a zero sequence voltage under a normal operation, Ė.sub.φ, is a power voltage of a ground fault phase, {dot over (U)}.sub.φ1 is the fault phase voltage in a range of [0, U.sub.φ0) after the external voltage source is applied, and U.sub.φ0 is the fault phase voltage before the external voltage source is applied.

2. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 1, wherein during the voltage reduction operation, a current İ injected by the external voltage source is measured and calculated, and a magnitude and a phase of the external voltage source {dot over (U)} are regulated to establish formula İ=Δ{dot over (U)}.Math.ΣY.sub.0, so that the arc of a fault point is suppressed, where ΣY.sub.0 is a zero sequence admittance to a ground when the non-effective ground system runs normally.

3. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 2, wherein the input voltage of the external voltage source comes from a secondary side voltage of the transformer of the non-effective ground system, and is identical to the fault phase power voltage in phase.

4. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 2, wherein a single-phase voltage regulator is installed in the external voltage source to regulate the amplitude of the voltage.

5. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 1, wherein during the voltage reduction operation, a damping rate $d=\frac{g}{\omega C}=\frac{U_{0}g}{U_0\omega C}=\frac{I_{0R}}{I_{0C}}=\frac{P_o}{Q_0}=\cot {\alpha }_0$ of the non-effective ground system or the damping rate of the ground fault line is measured and calculated; if the damping rate d is greater than a setting value, the magnitude and phase of the voltage {dot over (U)} output by the external voltage source are regulated, so that the fault phase voltage is further reduced to suppress the fault arc until d is smaller than or equal to the setting value, 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 ground, ω is an angular frequency of the system, C is a three-phase capacitance to ground, U.sub.0 is a zero sequence voltage, I.sub.0R is a zero sequence active current, 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 α.sub.0 is a zero sequence admittance angle.

6. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 5, wherein the setting value 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; and the coefficient K.sub.3 is in a range of (1, 5].

7. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 6, wherein the input voltage of the external voltage source comes from a secondary side voltage of the transformer of the non-effective ground system, and is identical to the fault phase power voltage in phase.

8. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 5, wherein the input voltage of the external voltage source comes from a secondary side voltage of the transformer of the non-effective ground system, and is identical to the fault phase power voltage in phase.

9. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 5, wherein a single-phase voltage regulator is installed in the external voltage source to regulate the amplitude of the voltage.

10. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 1, wherein during the voltage reduction operation of the distribution network, the zero sequence current of the ground fault line is measured; if the zero sequence current is greater than a setting value, the magnitude and phase of the voltage {dot over (U)} output by the external voltage source are regulated, 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 setting value, and safe operation of active voltage reduction of the ground fault phase is achieved.

11. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 10, wherein the input voltage of the external voltage source comes from a secondary side voltage of the transformer of the non-effective ground system, and is identical to the fault phase power voltage in phase.

12. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 10, wherein a single-phase voltage regulator is installed in the external voltage source to regulate the amplitude of the voltage.

13. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 1, wherein after the ground fault is detected, the application of the external voltage source continues for a period of time, then the external voltage source is disconnected, whether the ground fault exists is detected again, and if the fault does not exist, it is determined that the instantaneous ground fault has been extinguished to restore normal operation; otherwise, the external voltage source is applied again to continue the active voltage reduction operation of the ground fault phase.

14. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 13, wherein the input voltage of the external voltage source comes from a secondary side voltage of the transformer of the non-effective ground system, and is identical to the fault phase power voltage in phase.

15. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 13, wherein a single-phase voltage regulator is installed in the external voltage source to regulate the amplitude of the voltage.

16. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 1, wherein the external voltage source is a voltage source with adjustable amplitude and phase realized by power electronic components, or a voltage source output by an external single-phase transformer.

17. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 16, wherein the input voltage of the external voltage source comes from a secondary side voltage of the transformer of the non-effective ground system, and is identical to the fault phase power voltage in phase.

18. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 1, wherein the input voltage of the external voltage source comes from a secondary side voltage of the transformer of the non-effective ground system, and is identical to the fault phase power voltage in phase.

19. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 1, wherein a single-phase voltage regulator is installed in the external voltage source to regulate the amplitude of the voltage.

20. The safe operation method for voltage reduction arc suppression of the ground fault in the phase of the non-effective ground system according to claim 1, wherein a protection device is arranged in the output circuit of the external voltage source to prevent the equipment from being damaged by high current.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a principle diagram of safe operation of voltage reduction arc suppression of a ground fault phase of a non-effective ground distribution network using a Y/A 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 arc 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 safe operation of voltage reduction arc suppression 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 safe operation of voltage reduction arc suppression 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, Ė.sub.A, Ė.sub.B and Ė.sub.C are respectively three phases of power electromotive forces of a system, C.sub.0 is system capacitance to ground, r.sub.0 is system leakage resistance to ground; A.sub.s, B.sub.s and C.sub.s are non-effective ground system side windings of a Y/Δ wiring transformer, S is a switch, K is a protection device (an overcurrent protection device or a fuse), outgoing 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; 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. When a single-phase ground fault occurs, the ground fault resistance is R.sub.f, the fault phase voltage φ is {dot over (U)}.sub.φ0 (φ=A or B or C), an external voltage source {dot over (U)} is applied at the non-effective ground system side between a bus and the ground, or between a line and the ground, or between a neutral point and the ground, or between a shunting tap of the transformer T.sub.1 and the ground to cause simultaneous rise or drop of ground voltage of the entire distribution system, the variation is Δ{dot over (U)}.sub.0, then {dot over (U)}={dot over (U)}.sub.1+Δ{dot over (U)}.sub.0, and the variation of zero sequence voltage Δ{dot over (U)}.sub.0 may also be calculated from formula Δ{dot over (U)}.sub.0={dot over (U)}.sub.03−{dot over (U)}.sub.01 or Δ{dot over (U)}.sub.0={dot over (U)}.sub.φ1−Ė.sub.φ; where {dot over (U)}.sub.1 is normal voltage of an access point when a normal grid voltage source is not connected, {dot over (U)}.sub.03 is zero sequence voltage after active voltage reduction, {dot over (U)}.sub.01 is zero sequence voltage under normal operation, Ė.sub.φ is power voltage of a ground fault phase, {dot over (U)}.sub.φ1 is fault phase voltage in a range of [0, U.sub.φ0) after the external voltage source is applied, and U.sub.φ0 is fault phase voltage before the external voltage source is applied.

(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 current İ injected by the voltage source {dot over (U)} is:

(10) $\stackrel{.}{I}=j3{\stackrel{.}{U}}_0\omega C_0+\frac{3{\stackrel{.}{U}}_0}{r_0}+\frac{{\stackrel{.}{U}}_C}{R_f}+\frac{{\stackrel{.}{U}}_0}{Z}={\stackrel{.}{U}}_0\Sigma Y_0+{\stackrel{.}{U}}_0{Y}_f$

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

(12) $\Sigma Y_0=\frac{1}{Z}+j3\omega C_0+\frac{3}{r_0}=Y+j\omega C+g,$
the grounding admittance of the neutral point is

(13) $Y=\frac{1}{Z},$
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 conductance to ground in fault point is

(15) $Y_f=\frac{1}{R_f},$
and {dot over (U)}.sub.0 is 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 ΔU.sub.0.sup.&; and considering the admittance to ground Y.sub.f=0 of a fault point after fault arc suppression, equation (1) may be simplified as:
İ=Δ{dot over (U)}.Math.ΣY.sub.0=({dot over (U)}.sub.03−{dot over (U)}.sub.01).Math.ΣY.sub.0=({dot over (U)}.sub.3−{dot over (U)}.sub.1).Math.ΣY.sub.0  (2)

(17) Thus, during voltage reduction operation, the current İ injected by the voltage source is measured and calculated, and the magnitude and phase of the external voltage source {dot over (U)} are regulated to establish formula İ=Δ{dot over (U)}.Math.ΣY.sub.0, so that the arc of the fault point is suppressed, where ΣY.sub.0 is a zero sequence admittance to ground when the non-effective ground system runs normally.

(18) From the foregoing, after a ground fault occurs in the system, an external adjustable voltage source is adopted to realize active voltage reduction arc suppression, and the voltage output by the voltage source is uniquely determined by a target value of fault phase voltage reduction, which can be achieved by applying the external voltage source to a bus at the non-effective ground system side, or to a line, or to a neutral point, or to a shunting tap of a non-effective ground system side winding of the transformer.

(19) The following further discusses a fault phase voltage reduction operation range of fault arc 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, set the maximum operating voltage amplitude of the fault phase to ensure fault phase arc suppression is CC″, the condition of fault phase arc 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 overhigh voltage of a non-fault phase, the non-fault phase voltage 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 arc suppression by measuring a damping rate. As shown in FIG. 4, during voltage reduction arc suppression, the damping rate of the system is calculated by measuring the zero sequence current İ.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 İ.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) $d=\frac{g}{\omega C}=\frac{U_{0}g}{U_0\omega C}=\frac{I_{0R}}{I_{0C}}=\frac{P_o}{Q_0}=\cot {\alpha }_0,$
and the setting value 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 setting value, the magnitude and phase of the voltage {dot over (U)} output by the voltage source are regulated, so that the fault phase voltage is further reduced to suppress the fault arc until d is smaller than or equal to the setting value, 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, ω 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 α.sub.0 is zero sequence admittance angle.

(23) 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:

(24) As shown in FIG. 5, in a non-effective ground 10 kV distribution network, E.sub.A, E.sub.B and E.sub.C are respectively three phases of power electromotive forces of a system, 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 kΩ, the capacitance to ground of line C.sub.0 is 8.36 uF, K is a protection device (an overcurrent protection device or a fuse), the zero sequence current setting value of the ground fault line is set to 10 A, and the ground impedance Z of the neutral point N is j121Ω; A.sub.1, B.sub.1, C.sub.1, A.sub.2, B.sub.2 and C.sub.2 are non-effective ground system side windings of a Z-type ground transformer, KM1, KM2 and KM3 are contactors, outgoing 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, the non-effective ground system side windings of the transformer are connected in a Z shape, a neutral point N is led out from the other ends and then the windings are grounded through an impedance Z; a.sub.1, b.sub.1 and c.sub.1 are low-voltage side windings of the Z-type ground transformer, the low-voltage side windings are star-connected, the leading-out terminals are represented by a, b, c and n, T.sub.1 is the Z-type ground transformer that can provide a neutral point, T.sub.2 is a voltage source output by an external single-phase transformer and connected between the neutral point and the ground, the input voltage of the single-phase transformer comes from the secondary side voltage of the ground transformer T.sub.1 and is identical to a fault phase power voltage in phase, and the amplitude of the output voltage is adjustable. If a single-phase ground fault occurs in phase C, the ground fault resistance is represented by R.sub.f, R.sub.f=1 kΩ, and {dot over (U)}.sub.C1 is the fault phase voltage. After the fault occurs, before the external voltage source is applied, the fault phase voltage is measured as U.sub.C0=2.60 kV, and the critical voltage for sustaining continuous burning of a ground arc is 1.90 kV. At this time, the external adjustable voltage source {dot over (U)} is applied between the neutral point of the non-effective ground distribution network and the ground, the normal voltage U.sub.1 of an access point when the normal grid voltage source is not connected is 0 V, then it is obtained:
{dot over (U)}={dot over (U)}.sub.1−Ė.sub.C+{dot over (U)}.sub.C1  (3).

(25) The external voltage source {dot over (U)} is regulated to reduce the fault phase voltage U.sub.C1 to be lower than a voltage for continuous burning of the ground arc, that is, U.sub.C1<1.90 kV, thus achieving ground fault arc blowout. In this example, if the fault phase voltage U.sub.C1 is reduced for arc suppression at 1.82 kV, the amplitude of voltage output by the voltage source may be adjusted first as U=3.95 kV, and then the contactor KM2 is closed to reduce the fault phase voltage to 1.82 kV, which satisfies the fault phase voltage operating range [0, 2.60 kV). At this time, the non-fault phase voltage is 8.51 kV, which is smaller than the line voltage 10 kV, thereby realizing arc suppression of the ground fault phase. Meanwhile, the non-fault phase voltage does not rise to the line voltage, thereby realizing safe operation of voltage reduction arc suppression.

(26) During voltage reduction operation, the zero sequence current of a ground fault line is measured; if the zero sequence current is greater than a setting value 10 A, the magnitude of voltage output by the voltage source is continuously regulated, 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 setting value 10 A, and safe operation of voltage reduction arc suppression of the ground fault phase is achieved.

(27) As shown in FIG. 6, in a 20 kV non-effective ground generator, the three phases of power are respectively E.sub.A=E.sub.B=E.sub.C=20/√{square root over (3)}kV, the leakage resistance to ground r.sub.0 of the generator stator is 20 kΩ, the capacitance to ground C.sub.0 of the generator stator is 1.81 uF, K is a protection device (an overcurrent protection device or a fuse), the ground impedance Z of the neutral point N is j600Ω, a ground fault occurs in phase C of the distribution network, and the ground fault resistance R.sub.f is 2 kΩ. After the fault occurs, before an external voltage source is applied, the measured fault phase voltage U.sub.C0 is 2.76 kV, and the critical voltage for sustaining continuous burning of a ground arc is 2.20 kV. At this time, the external adjustable voltage source is applied between the neutral point of the non-effective ground generator and the ground, the normal voltage U.sub.1 of an access point when the normal generator voltage source is not connected is 0 V, then {dot over (U)}={dot over (U)}.sub.1−Ė.sub.C+{dot over (U)}.sub.C1, and the external voltage source {dot over (U)} is regulated to reduce the fault phase voltage U.sub.C1 to be lower than a voltage for continuous burning of the ground arc, that is, U.sub.C1<2.20 kV, thus achieving ground fault arc blowout. In this example, if the fault phase voltage U.sub.C1 is reduced for arc suppression at 2.13 kV, the amplitude of voltage output by the voltage source may be adjusted first as U=9.41 kV, and then the contactor KM2 is closed to reduce the fault phase voltage to 2.13 kV, which satisfies the fault phase voltage operating range [0, 2.76 kV). At this time, the non-fault phase voltage is 18.27 kV, which is smaller than the line voltage 20 kV, thereby realizing arc suppression of the ground fault phase. Meanwhile, the non-fault phase voltage does not rise to the line voltage, thereby realizing safe operation of voltage reduction arc suppression.

(28) As an improvement of the present embodiment, after the ground fault is detected, the application of the external voltage source continues for a period of time, then the voltage source is disconnected, whether the ground fault exists is detected again, and if the fault does not exist, it is determined that the instantaneous ground fault has been extinguished to restore normal operation; otherwise, the external voltage source is applied again to continue the active voltage reduction operation of the ground fault phase; where the duration of the external voltage source is (0.1 s, 60 s).

(29) In order to verify the feasibility of the safe operation method for voltage reduction arc suppression of a ground fault phase of a non-effective ground system according to the present invention, the safe operation method for voltage reduction arc suppression of the ground fault phase of a 10 kV non-effective ground distribution network shown in FIG. 1 was simulated and analyzed in PSCAD simulation software: the simulation duration is 0.12 s, a single-phase ground fault occurred at 0.04 s in the system; the switch is turned on at 0.08 s to apply the external voltage source between the neutral point and the ground; the simulation results before and after the single-phase ground fault (ground fault resistance 1 kΩ) of the non-effective ground distribution system is shown in Table 1.

(30) TABLE-US-00001 TABLE 1 Non-fault phase Voltage Fault current (A) Fault phase voltage (kV) voltage (kV) source During After voltage During After voltage After voltage voltage fault reduction fault reduction reduction (kV) 2.71 0 2.60 1.82 8.51 3.95

(31) It can be known by comprehensive analysis on the data in Table 1 that, after the ground fault occurs and the voltage source is applied, the fault phase voltage is reduced to operate at 1.82 kV, in a range of [0, U.sub.φ0), U.sub.ϕ0=2.60 kV is the fault phase voltage before the external voltage source is applied, and the non-fault phase voltage is 8.51 kV at this time, which is smaller than the line voltage 10 kV. The simulation results show that the present invention not only reduces the fault phase voltage to meet the requirements of long-time non-stop safe operation, but also reduces the risk of non-fault phase insulation breakdown to greatly improve the reliability and safety of power supply.