METHOD TO DETECT UTILITY DISTURBANCE AND FAULT DIRECTION

Abstract

A method for detecting a voltage disturbance in a utility having a micro-grid. The method transforms current and voltage measurements into first and second voltage values and first and second current values in a stationary reference frame, and transforms the first and second voltage values and the first and second current values to third and fourth voltage values and third and fourth current values in a rotating reference frame, where the third voltage value defines an average magnitude of the three-phase power signals. The method multiplies the third current value by the third voltage value to obtain an instantaneous power value and multiplies the fourth current value by the third voltage value to obtain an instantaneous volt-ampere reactive (VAR) value. The method opens the switch if the VAR value has a magnitude above a predetermined value and a direction indicating the fault is outside of the micro-grid in the network.

Claims

1. A method for detecting a voltage disturbance in an electrical power distribution network that includes an electrical system having one or more power sources that can be disconnected from the network by a disconnect switch provided in an electrical line, where the network provides three-phase electrical AC power signals to the electrical system, said method opening the disconnect switch in response to detecting the voltage disturbance only if the voltage disturbance is in the network outside of the electrical system, said method comprising: reading instantaneous voltage measurements of each of the three-phase power signals on the electrical line; reading instantaneous current measurements of each of the three-phase power signals on the electrical line; detecting whether a fault is occurring in the distribution network by using the voltage measurements to detect a voltage disturbance on the electrical line; transforming the voltage measurements into first and second voltage values in a stationary reference frame; transforming the first and second voltage values in the stationary reference frame to third and fourth voltage values in a rotating reference frame, where the third voltage value defines an average magnitude of the three-phase power signals and the fourth voltage value provides an indication of whether the third voltage value is locked to a network voltage; correcting the fourth voltage value over time to maintain the third voltage value locked to the network voltage; transforming the current measurements into first and second current values in the stationary reference frame; transforming the first and second current values in the stationary reference frame to third and fourth current values in a rotating reference frame; multiplying the third current value by the third voltage value to obtain an instantaneous power value; multiplying the fourth current value by the third voltage value to obtain an instantaneous volt-ampere reactive (VAR) value; and opening the switch if the VAR value has a magnitude above a predetermined value and a sign indicating the voltage disturbance is outside of the electrical system in the network.

2. The method according to claim 1 wherein detecting a voltage disturbance includes calculating a sliding window root mean squared (RMS) voltage for each three-phase power signal over a first predetermined sample period using the instantaneous voltage measurements.

3. The method according to claim 1 wherein transforming the voltage measurements into first and second voltage values and transforming the current measurements into first and second current values includes employing a Clarke transformation, and wherein transforming the first and second voltage values into third and fourth voltage values and transforming the first and second current values includes into third and fourth current values includes employing a Park transformation.

4. The method according to claim 3 wherein transforming the first and second voltage values and transforming the first and second current values includes using a sine and cosine of a frequency correction angle.

5. The method according to claim 4 wherein employing the Park and Clark transformations includes using the equations: $V_{\mathrm{ds}}=\frac{2}{3}.Math.V_a-\frac{1}{3}.Math.\left(V_b+V_c\right),.Math.V_{\mathrm{qs}}=\frac{1}{\sqrt{3}}.Math.\left(V_b-V_c\right),.Math.V_{\mathrm{dr}}=V_{\mathrm{ds}}.Math.\cos \left(\right)+V_{\mathrm{qs}}.Math.\sin \left(\right),.Math.V_{\mathrm{qr}}=V_{\mathrm{qs}}.Math.\cos \left(\right)-V_{\mathrm{ds}}.Math.\sin \left(\right),.Math.I_{\mathrm{ds}}=\frac{2}{3}.Math.I_a-\frac{1}{3}.Math.\left(I_b+I_c\right),.Math.I_{\mathrm{qs}}=\frac{1}{\sqrt{3}}.Math.\left(I_b-I_c\right),.Math.I_{\mathrm{dr}}=I_{\mathrm{ds}}.Math.\cos \left(\right)+I_{\mathrm{qs}}.Math.\sin \left(\right),.Math.I_{\mathrm{qr}}=I.Math..Math.\cos \left(\right)-I_{\mathrm{ds}}.Math.\sin \left(\right),$ where V.sub.a, V.sub.b and V.sub.c are the instantaneous voltage measurements of the three-phase power signals, I.sub.a, I.sub.b and I.sub.c are the instantaneous current measurements of the three-phase power signals, V.sub.ds is the first voltage value, V.sub.qs is the second voltage value, V.sub.dr is the third voltage value, V.sub.qr is the fourth voltage value, I.sub.ds is the first current value, I.sub.qs is the second current value, I.sub.dr is the third current value, I.sub.qr is the fourth current value and is the frequency correction angle.

6. The method according to claim 4 wherein correcting the fourth voltage value over time includes providing the fourth voltage value to a proportional-integral (P-I) regulator that generates a correction frequency that changes a base frequency of the network, where the frequency correction angle is determined from the changed base frequency.

7. The method according to claim 1 further comprising filtering the power value and the VAR value to remove transients therefrom.

8. The method according to claim 1 wherein the electrical system is a critical load or a micro-grid.

9. The method according to claim 1 wherein opening the switch further includes looking at the current direction of the power signal.

10. A method for detecting a voltage disturbance in an electrical power distribution network that includes an electrical system having one or more power sources that can be disconnected from the network by a disconnect switch provided in an electrical line, where the network provides three-phase electrical AC power signals to the electrical system, said method opening the disconnect switch in response to detecting the voltage disturbance only if the voltage disturbance is in the network outside of the electrical system, said method comprising: reading instantaneous voltage measurements of each of the three-phase power signals on the electrical line; reading instantaneous current measurements of each of the three-phase power signals on the electrical line; transforming the voltage measurements into first and second voltage values in a stationary reference frame using a Clarke transformation; transforming the first and second voltage values in the stationary reference frame to third and fourth voltage values in a rotating reference frame using a Park transformation, where the third voltage value defines an average magnitude of the three-phase power signals and the fourth voltage value provides an indication of whether the third voltage value is locked to a network voltage; transforming the current measurements into first and second current values in the stationary reference frame using a Clarke transformation; transforming the first and second current values in the stationary reference frame to third and fourth current values in a rotating reference frame using a Park transformation; multiplying the third current value by the third voltage value to obtain an instantaneous power value; multiplying the fourth current value by the third voltage value to obtain an instantaneous volt-ampere reactive (VAR) value; and opening the switch if the VAR value has a magnitude above a predetermined value and a sign indicating the voltage disturbance is outside of the electrical system in the network.

11. The method according to claim 10 wherein transforming the first and second voltage values and transforming the first and second current values includes using a sine and cosine of a frequency correction angle.

12. The method according to claim 11 further comprising correcting the fourth voltage value over time to maintain the third voltage value locked to the network voltage, wherein correcting the fourth voltage value over time includes providing the fourth voltage value to a proportional-integral (P-I) regulator that generates a correction frequency that changes a base frequency of the network, where the frequency correction angle is determined from the changed base frequency.

13. The method according to claim 10 wherein the electrical system is a critical load or a micro-grid.

14. A detection system for detecting a voltage disturbance in an electrical power distribution network that includes an electrical system having one or more power sources that can be disconnected from the network by a disconnect switch provided in an electrical line, where the network provides three-phase electrical AC power signals to the electrical system, said method opening the disconnect switch in response to detecting the voltage disturbance only if the voltage disturbance is in the network outside of the electrical system, said detection system comprising: means for reading instantaneous voltage measurements of each of the three-phase power signals on the electrical line; means for reading instantaneous current measurements of each of the three-phase power signals on the electrical line; means for detecting whether a fault is occurring in the distribution network by using the voltage measurements to detect a voltage disturbance on the electrical line; means for transforming the voltage measurements into first and second voltage values in a stationary reference frame; means for transforming the first and second voltage values in the stationary reference frame to third and fourth voltage values in a rotating reference frame, where the third voltage value defines an average magnitude of the three-phase power signals and the fourth voltage value provides an indication of whether the third voltage value is locked to a network voltage; means for correcting the fourth voltage value over time to maintain the third voltage value locked to the network voltage; means for transforming the current measurements into first and second current values in the stationary reference frame; means for transforming the first and second current values in the stationary reference frame to third and fourth current values in a rotating reference frame; means for multiplying the third current value by the third voltage value to obtain an instantaneous power value; means for multiplying the fourth current value by the third voltage value to obtain an instantaneous volt-ampere reactive (VAR) value; and means for opening the switch if the VAR value has a magnitude above a predetermined value and a sign indicating the voltage disturbance is outside of the electrical system in the network.

15. The detection system according to claim 14 wherein the means for detecting a voltage disturbance calculates a sliding window root mean squared (RMS) voltage for each three-phase power signal over a first predetermined sample period using the instantaneous voltage measurements.

16. The detection system according to claim 14 wherein the means for transforming the voltage measurements into first and second voltage values and the means for transforming the current measurements into first and second current values employ a Clarke transformation, and wherein the means for transforming the first and second voltage values into third and fourth voltage values and the means for transforming the first and second current values includes into third and fourth current values employ a Park transformation.

17. The detection system according to claim 14 wherein the means for transforming the first and second voltage values and the means for transforming the first and second current values use a sine and cosine of a frequency correction angle.

18. The detection system according to claim 17 wherein the means for correcting the fourth voltage value over time provides the fourth voltage value to a proportional-integral (P-I) regulator that generates a correction frequency that changes a base frequency of the network, where the frequency correction angle is determined from the changed base frequency.

19. The detection system according to claim 14 wherein the electrical system is a critical load or a micro-grid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a simplified schematic diagram of an electrical power distribution network including a micro-grid;

[0013] FIG. 2 is a schematic block diagram showing a method for determining the direction of fault current through a disconnect switch in the network shown in FIG. 1; and

[0014] FIG. 3 is a flow chart diagram showing a process for determining whether the disconnect switch should be opened or closed based on fault current direction.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0015] The following discussion of the embodiments of the disclosure directed to a method for detecting a voltage disturbance in a utility and the direction of fault current between a micro-grid and the utility is merely exemplary in nature, and is in no way intended to limit the disclosure or its applications or uses.

[0016] FIG. 1 is a schematic diagram of an electrical power distribution network 10, sometimes referred to herein as a utility. The network 10 is intended to represent any electrical power distribution system or network of any size and configuration that provides electrical power from any number or type of power plants (not shown) over any suitable distance on any type of transmission line (not shown) to electrical substations (not shown) to be distributed on feeder lines (not shown) to any suitable load (not shown).

[0017] The network 10 includes an AC power source 12, such as an electrical substation, that provides power on an electrical line 14 to the loads within the network 10. The line 14 is intended to represent three electrical lines each carrying one phase of a three-phase electrical service, where each phase signal on the line 14 has a line inductance represented by inductor 16 and a line resistance represented by resistor 18. The network 10 includes a UPS system or micro-grid 22 of the type referred to above having an electrical line 26 that receives power from the line 14 through a disconnect switch 24, where the line 26 also represents a line for each of the three-phase power signals and includes an inductance represented by inductor 28 and a resistance represented by resistor 30. The micro-grid 22 includes a micro-grid power source 32 intended to represent any power generation device for any application, such as photovoltaic cells, battery banks, wind farms, diesel generators, etc. The micro-grid 22 includes one or more loads that receive electrical power from the source 12 when the switch 24 is closed and/or from the micro-grid power source 32, where the load or loads are represented by an inductor 34 and a resistor 36.

[0018] Three current transformers (CTs) 40 provided in the line 14 measure the current of the three-phase AC power signals and three potential transformers (PTs) 42 provided in the line 14 measure the voltage of the three-phase power signals, and those current and voltage measurements are provided to a controller 44. As will be discussed in detail below, the controller 44 controls the position of the switch 24 so that if a voltage disturbance is detected by the controller 44 as a result of a fault in the utility, the controller 44 will open the switch 24 only if it determines that the fault is in the utility and not in the micro-grid 22. If the fault is in the micro-grid 22 it is desirable to maintain the switch 24 closed so that the various over-current protection devices and switches (not shown) in the micro-grid 22 can clear the fault without affecting all of the loads in the micro-grid 22 or the loads in the utility.

[0019] As discussed herein, the controller 44 simultaneously detects a voltage disturbance caused by a fault in the network 10 and determines whether the fault is outside or inside of the micro-grid 22 by determining the fault direction relative to the switch 24. To detect the voltage disturbance, in one non-limiting embodiment, the controller 44 employs sliding window one-half cycle RMS voltage calculations, such as discussed above in the '246 patent, to determine whether any of the three-phase voltage measurement signals V.sub.a, V.sub.b and V.sub.c from the potential transformers 42 has sagged some predetermined maximum or minimum amount to detect the fault, although other techniques may be equally applicable. In one embodiment, the various calculations discussed herein are performed at 4800 Hz or 4800 times per second. For a 60 Hz AC signal, there would thus be 80 sample points or calculations per AC cycle. Since the sliding window is over a one-half cycle, each calculation at each sample point uses the last forty voltage measurements, where the algorithm squares each voltage measurement, adds the forty squared voltage measurements, divides the added voltage measurements by forty and takes the square root of that value to give the RMS voltage values. Alternately, the algorithm could compare added forty square values without dividing by forty.

[0020] FIG. 2 is a schematic block diagram of a system 50 showing the operation of an algorithm operating in the controller 44 that uses the voltage and current measurements from the current transformers 40 and the potential transformers 42 to determine a change in power and in volt-ampere reactive (VAR) units to determine if a fault that causes a detected voltage disturbance is outside of the micro-grid 22 in the network 10 or inside the micro-grid 22. As is known in the art, a VAR unit defines reactive power, specifically power where the current and voltage are not in phase in an AC power signal, where VARs can define either the imaginary part of the apparent power or the power flowing into a reactive load.

[0021] The system 50 includes a voltage transformation side 52 and a current transformation side 54, where the voltage transformation side 52 receives the three-phase voltage measurement signals V.sub.a, V.sub.b and V.sub.b from the potential transformers 42 and the current transformation side 54 receives the three-phase current measurement signals I.sub.a, I.sub.b and I.sub.b from the current transformers 40. The voltage measurement signals V.sub.a, V.sub.b and V.sub.b are provided to a Clarke transformation block 58 that converts the three-phase voltage signals V.sub.a, V.sub.b and V.sub.b into two-phase direct axis (d) and quadrature axis (q) voltage signals V.sub.ds and V.sub.qs in a stationary reference frame in a manner well understood by those skilled in the art. The two-phase voltage signals V.sub.ds and V.sub.qs are then provided to a Park transformation block 60 that converts the two-phase voltage signals V.sub.ds and V.sub.qs in the stationary reference frame to two-phase direct axis (d) and quadrature axis (q) voltage signals V.sub.dr and V.sub.qr in a rotating reference frame also in a manner well understood by those skilled in the art, where V.sub.dr is the average magnitude of the three-phase voltage signals V.sub.a, V.sub.b and V.sub.b. The Park transformation block 60 uses the sine of a frequency correction angle provided by block 66 and the cosine of the angle provided by block 68 to convert to the rotational reference frame.

[0022] Likewise, the current measurement signals I.sub.a, I.sub.b and I.sub.b are provided to a Clarke transformation block 62 that converts the three-phase current signals I.sub.a, I.sub.b and I.sub.b into two-phase direct axis (d) and quadrature axis (q) current signals I.sub.ds and I.sub.qs in a stationary reference frame. The two-phase current signals I.sub.ds and I.sub.qs are then provided to a Park transformation block 64 that converts the two-phase current signals I.sub.ds and I.sub.qs in the stationary reference frame to two-phase direct axis (d) and quadrature axis (q) current signals I.sub.dr and I.sub.qr in a rotating reference frame. Likewise, the Park transformation block 64 uses the sine of the angle provided by block 70 and the cosine of the angle provided by block 72 to convert to the rotational reference frame. The transformations performed in the blocks 60, 62, 64 and 66 can be defined by equations (1)-(8) below.

[00001]$\begin{array}{cc}{V}_{\mathrm{ds}}=\frac{2}{3}.Math.V_a-\frac{1}{3}.Math.\left(V_b+V_c\right),& \left(1\right)\\ V_{\mathrm{qs}}=\frac{1}{\sqrt{3}}.Math.\left(V_b-V_c\right),& \left(2\right)\\ V_{\mathrm{dr}}=V_{\mathrm{ds}}.Math.\cos \left(\right)+V_{\mathrm{qs}}.Math.\sin \left(\right),& \left(3\right)\\ V_{\mathrm{qr}}=V_{\mathrm{qs}}.Math.\cos \left(\right)-V_{\mathrm{ds}}.Math.\sin \left(\right),& \left(4\right)\\ I_{\mathrm{ds}}=\frac{2}{3}.Math.I_a-\frac{1}{3}.Math.\left(I_b+I_c\right),& \left(5\right)\\ I_{\mathrm{qs}}=\frac{1}{\sqrt{3}}.Math.\left(I_b-I_c\right),& \left(6\right)\\ I_{\mathrm{dr}}=I_{\mathrm{ds}}.Math.\cos \left(\right)+I_{\mathrm{qs}}.Math.\sin \left(\right),& \left(7\right)\\ I_{\mathrm{qr}}=I.Math..Math.\cos \left(\right)-I_{\mathrm{ds}}.Math.\sin \left(\right).& \left(8\right)\end{array}$

[0023] The phase of the voltage value V.sub.dr in the rotating frame needs to be synchronized or locked to the phase of the voltage on the line 14. When this occurs, the voltage value V.sub.dr should be about 1 per unit (pu) and the voltage value V.sub.qr should be about 0 at the nominal voltage. However, the transformation of the voltage value V.sub.ds to the rotating frame may cause loss of synchronization. To maintain V.sub.qr at about 0, a proportional-integral (P-I) regulator 76 operates to change the frequency correction angle to force the voltage value V.sub.qr back to zero as it drifts therefrom. Particularly, the voltage value V.sub.qr is applied to a proportional gain block 78 having a gain K.sub.p and an integral gain block 80 having a gain K.sub.i, where if the voltage value V.sub.qr increases or decreases from zero, proportional and integral error correction terms are generated. The integral term from the block 80 is integrated at block 82, and the proportional and integral terms are added together in a summation junction 84 to provide a frequency change value d provided to a summation junction 86 that also receives a base frequency value , for example, 60 Hz, so that the output of the summation junction 86 will be a frequency correction that is converted to the frequency offset angle based on time that maintains the voltage value V.sub.qr at zero.

[0024] If the current value I.sub.dr has a magnitude I.sub.qr is zero, then the power signal on the line 14 is only real power, and the current value I.sub.dr will be the magnitude of the real power current, but the current value I.sub.qr representing the current in the VAR direction will be zero. The voltage value V.sub.dr is multiplied by the current value I.sub.dr in multiplication block 90 and is multiplied by the current value I.sub.qr in multiplication block 92. If the voltage and current on the line 14 are in phase, then the output of the multiplication block 90 will provide an instantaneous power value on line 98 after being filtered by a filter 96 that removes transients. If the current value I.sub.qr has a magnitude, then that value multiplied by the voltage V.sub.qr provides an instantaneous VAR value on line 102 after being filtered by a filter 100 that removes transients. If V.sub.dr is not zero, then the full equation is P=V.sub.dr*I.sub.dr+V.sub.qr*I.sub.qr and Q=V.sub.dr*I.sub.drV.sub.qr*I.sub.qr.

[0025] The power and VAR values allow the controller 44 to determine the direction of the fault. The magnitude of the VAR value is analyzed to determine whether it is higher than the rating of the load, where if the current is not above this value, then fault current does not exist. By converting the three-phase voltages and currents to two phases, an instantaneous calculation of power and VARs can be provided. Therefore, depending on whether the VAR value is positive or negative identifies which direction the current is flowing through the switch 24, and thus the direction of the fault. Particularly, a positive VAR value indicates that the current is flowing through the switch 24 into the micro-grid 22 and a negative VAR value indicates that the current is flowing from the micro-grid 22 back to the utility.

[0026] The following is pseudo-code for the operations described in reference to the system 50.

TABLE-US-00001 LineImpedanceX = 0.06; // set the line impedance due to inductance to 6% on a 1PU current basis LineImpedanceR = 0.02; // set the line impedance due to resistance to 2% LineImpedance = (LineImpedanceX{circumflex over ()}2 + LineImpedanceR{circumflex over ()}2){circumflex over ()}0.5; // complex sum of the impedances // Put in the overload capability at the moment. This can be a calculated // value, or may be a fixed value. OverLoadX = 1.0; // shown as a fixed value for simplicity OverLoadR = 0.1; // shown as a fixed value for simplicity // start with the voltage Clarke transform using the measured // instant line to neutral voltages Va, Vb, and Vc // Scaled with Vdr = 1 at 100% voltage Vds = (2/3 * Va) (1/3 * (Vb + Vc)); Vqs = 1/3{circumflex over ()}.5 * (Vb Vc); // Now do the Park transform using Theta of the PI regulator shown in the diagram Vdr = (Vds * Cos(Theta)) + (Vqs * Sin(Theta)); Vqr = (Vqs * Cos(Theta)) (Vds * Sin(Theta)); // Current Clarke transform using the measured // instant line currents Ia, Ib, and Ic // scaled so an output of 1 is 1 PU current as used to calculate impedance Ids = (2/3* Ia) (1/3 * (Ib + Ic)); Iqs = 1/3{circumflex over ()}.5 * (Ib Ic); // Now do the Park transform using Theta Idr = (Ids * Cos(Theta)) + (Iqs * Sin(Theta)); Iqr = (Iqs * Cos(Theta)) (Ids * Sin(Theta)); FilteredVdr = Lowpass(Vdr); // Lowpass filter of 1 to 100 seconds typical FilteredIdr = Lowpass(Idr); // Lowpass filter with same time constant as Vdr FilteredIqr = Lowpass(Iqr); // Lowpass filter with same time constant as Vdr DeltaVdr = FilteredVdr Vdr; // this is the change in Vdr DeltaIdr = FilteredIdr Idr; // this is the change in Idr DeltaIqr = FilteredIqr Iqr; // this is the change in Iqr DvFromIdr = DeltaIdr * LineImpedanceR; // expected voltage drop from real power increase in load DvFromIqr = DeltaIqr * LineImpedanceS; // expected voltage drop from reactive power increase in load DvTotal = DvFromIdr + DvFromIqr; // total expected voltage drop // here is the logic to determine if a fault is downstream. // VaRms is a half cycle sliding window RMS calculation FaultIsDownstream = False; // if any phase is below 90% of nominal voltage and // the Microgrid or UPS cannot supply the increased Power or VAR load if ( ((VaRms < 0.9) (VbRms < 0.9) (VcRms < 0.9)) && ((DeltaIdr > OverLoadR) (DeltaIqr > OverLoadX)) ) { if ( DvTotal > (0.5 * DeltaVdr) ) // if the voltage sag can be attributed at least 50% // to increase in power or Vars in the load { FaultIsDownstream = True; // Do not open the switch between the utility and the load

[0027] FIG. 3 is a flow chart diagram 110 showing the general operation of the algorithm discussed above for detecting a voltage disturbance and then determining whether to open the switch 24 if the voltage disturbance is caused by a fault in the utility or keep the switch 24 closed if the voltage disturbance is caused by a fault in the micro-grid 22. At box 112, the micro-grid 22 is connected to the utility and the switch 24 is closed. Decision diamond 114 determines whether there is a voltage disturbance, and if not the switch 24 remains closed. If a voltage disturbance is detected at the decision diamond 114, then the algorithm determines whether the disturbance is caused by a fault in the micro-grid 22 at decision diamond 116 in the manner discussed above with reference to FIG. 2. If the algorithm determines that the fault is in the micro-grid 22 at the decision diamond 116, then the switch 24 is maintained closed at the box 112. If the algorithm determines that the fault is in the utility and not in the micro-grid 22, then the algorithm opens the switch 24 at box 118, and operates the UPS or micro-grid source to maintain the loads in the micro-grid 22 at box 120. The algorithm then determines whether the fault has been cleared from the utility at decision diamond 122, and if not, returns to the box 120 to keep the micro-grid 22 connected to the source 32. If the fault is cleared from the utility at the decision diamond 122, then the algorithm closes the switch 24 and returns to normal operation at box 124.

[0028] The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.