Method and device for detecting a fault in an electrical network

Abstract

The invention relates to a method for detecting a fault in an electrical network (1) through which an AC current flows, the method comprising a step of acquiring three samples (S.sub.1, S.sub.2, S.sub.3) of a sinusoidal signal (S) that is representative of the current flowing in the network (1), the acquisition of each sample being spaced apart by a fixed sampling time (T), a step of calculating an amplitude (A) of the signal (S), the calculation of the amplitude (A) depending solely on the three acquired samples (S.sub.1, S.sub.2, S.sub.3) and being independent of the sampling time (T), a step of determining a fault if the calculated amplitude (A) is above a first predetermined threshold or if the calculated amplitude (A) is below a second predetermined threshold.

Claims

1. A method for detecting and protecting against a fault in an electrical network through which an AC current of nominal frequency flows, the method comprising: acquiring, with an analog sensor, a sinusoidal signal that is representative of the current flowing in the network; filtering, with an analog filter, the sinusoidal signal; sampling the filtered sinusoidal signal with a fixed sampling time to provide three sample measurement values S.sub.1, S.sub.2, and S.sub.3, the acquisition of each sample being spaced apart by the sampling time, the sampling time corresponding to a sampling frequency being more than or equal to three times the nominal frequency, the sample measurement value S.sub.2 being acquired intermediately in time between the sample measurement values S.sub.1 and S.sub.3; calculating an amplitude of the signal, the calculation of the amplitude depending solely on the three acquired samples, being performed using a quotient formed from the three acquired sample measurement values, and being independent of the sampling time, wherein when an absolute value of S.sub.2 is below or equal to a minimum threshold, during the step of calculating, the amplitude of the signal remains equal to a preceding calculated value; determining the fault if the calculated amplitude is above a first predetermined threshold or if the calculated amplitude is below a second predetermined threshold; and controlling the network to protect against the fault by at least one of opening a circuit in the network, limiting an imminent fault current, or reconfiguring the network to minimize a duration of a post-fault interruption.

2. The method for detecting and protecting against the fault according to claim 1, wherein the first threshold is set by a nominal amplitude of the signal to which a first margin or a first percentage of the nominal amplitude is added, and the second threshold is set by the nominal amplitude of the signal from which a second margin or a second percentage of the nominal amplitude is taken.

3. The method for detecting and protecting against the fault according to claim 1, in which the fixed sampling time corresponds to a sampling frequency being equal to 1800 Hz.

4. The method for detecting and protecting against the fault according to claim 1, in which the calculation of the amplitude uses the following formula: $A=\sqrt{S_2^2+{\left(\frac{S_3-S_1}{2.Math.\sin \left(\mathrm{Arc}\cos \left(\frac{S_1+S_3}{2.Math.S_2}\right)\right)}\right)}^2},$ where A is the amplitude.

5. The method for detecting and protecting against the fault according to claim 1, in which said controlling the network to protect against the fault comprises said reconfiguring the network to minimize the duration of the post-fault interruption.

6. A device for detecting and protecting against a fault in an electrical network through which an AC current of nominal frequency flows, the device comprising: circuitry configured to acquire, with an analog sensor, a sinusoidal signal that is representative of the current flowing in the network; filter, with an analog filter, the sinusoidal signal; sample the filtered sinusoidal signal with a fixed sampling time to provide three sample measurement values S.sub.1, S.sub.2, and S.sub.3, the acquisition of each sample being spaced apart by the sampling time, the sampling time corresponding to a sampling frequency being more than or equal to three times the nominal frequency, the sample measurement value S.sub.2 being acquired intermediately in time between the sample measurement values S.sub.1 and S.sub.3; calculate an amplitude of the signal, the calculation of the amplitude depending solely on the three acquired sample measurement values, being performed using a quotient formed from the three acquired sample measurement values, and being independent of the sampling time, wherein when an absolute value of S.sub.2 is below or equal to a minimum threshold, during the calculating, the amplitude of the signal remains equal to a preceding calculated value; determine the fault if the calculated amplitude is above a first predetermined threshold or if the calculated amplitude is below a second predetermined threshold; and control the network to protect against the fault by at least one of opening a circuit in the network, limiting an imminent fault current, or reconfiguring the network to minimize a duration of a post-fault interruption.

7. The device for detecting and protecting against the fault according to claim 6, wherein the circuitry is configured to store the three sample measurement values S.sub.1, S.sub.2, S.sub.3.

8. The device for detecting and protecting against the fault according to claim 6, wherein the circuitry comprises a protection relay and is configured to control the protection relay to protect the network against the fault.

9. The device for detecting and protecting against the fault according to claim 6, wherein the circuitry comprises an electrical switch connected to the network and is configured to control the electrical switch to protect the network against the fault.

10. The device for detecting and protecting against the fault according to claim 6, wherein the circuitry is configured to control the network to protect against the fault by said reconfiguring the network to minimize the duration of the post-fault interruption.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Other features and advantages will become more clearly apparent in the following description of particular embodiments of the invention, which are shown in the following appended figures:

(2) FIG. 1, already described above, shows an electrical network in which fault detection devices may be used;

(3) FIG. 2 shows an exemplary sampling of a signal that is representative of the current flowing in the electrical network;

(4) FIG. 3 shows, in a simplified manner, the detection method according to one preferred embodiment of the invention;

(5) FIGS. 4a to 4d illustrate an example with four diagrams showing a signal S and fault detection measurements according to the method of the invention and other methods;

(6) FIGS. 5a to 5d illustrate another example comparing the method of the invention and other methods.

DETAILED DESCRIPTION OF ONE EMBODIMENT

(7) FIG. 1 shows a diagram of a medium-voltage electrical distribution network 1 comprising a three-phase transformer 2, the secondary of which may comprise a common neutral conductor that is generally connected to ground by an impedance 3. The secondary is additionally connected to a main distribution line that supplies power to outgoing lines 4, 4, some of which may comprise, at the head, a circuit breaker or other switching device 5 protecting them.

(8) The outgoing lines 4, 4 are composed of overhead lines and/or underground cables and may be subject to various faults, and it is important that they are detected and located in order to mitigate the problems caused: power failure, deterioration of the withstand strength of the insulation materials, etc. Fault detection devices 6, 6i, 6, installed on the outgoing line 4 or line sections 4, serve to detect faults. Once a fault has been detected, the detection device 6 may comprise fault signalling means, such as a warning indicator 7, means for memorizing information relating, for example, to the presence of the fault, the type of fault and the duration of the fault. The detection device 6 may also comprise means for transmitting memorized fault information to a control station or a device for monitoring/controlling the electrical network 1.

(9) Such a device 6 may also be integrated in a unit for monitoring all or part 4, 4 of the electrical network 1, in particular in a protection relay 8 that is capable of controlling the opening of the contacts of the circuit breaker 5, or may be directly implanted in a circuit breaker so as to be able to quickly open the line 4 of the electrical network 1 in the event of a fault.

(10) Among these faults, the most common are single-phase faults, of short circuit type, that are located outside the source substation, in which one phase is in contact with ground, or an overhead cable breaks, especially in the event of bad weather. However, high capacitance values 9 may occur for example between the line conductors 4 and ground that result in large homopolar currents I.sub.0 flowing in the event of a ground fault 10.

(11) The homopolar current I.sub.0 is zero in the absence of faults. The term homopolar current I.sub.0 (or zero sequence current) is understood to be, to a potential factor of close to three, the vector sum of the various phase currents, or even the current corresponding to the instantaneous resultant of the phase currents, sometimes called residual current, that potentially corresponds to the ground fault current or to the leakage current. It should be noted that it is possible to escape this situation, in particular with a non-zero homopolar current/voltage, and the network may comprise another number of phases; additionally, the neutral regime does not have to be compensated.

(12) To detect the occurrence of a fault in an electrical network, it is known practice to measure, using sensors, at least one signal S that is representative of the current flowing in the portion of the network to be tested. This representative signal S may, for example, be the voltage between one of the phases and ground, the voltage between two phases, the current flowing in one of the phases or the current of the set of phases. In the absence of faults, the signal S is a sinusoidal signal of nominal amplitude A.sub.0 and nominal frequency f.sub.0, corresponding to the nominal frequency of the current flowing in the electrical network, e.g. 50 Hz or 60 Hz.

(13) In the presented embodiment, to detect a fault 10 in the line 4, the detection device 6 comprises an acquisition module comprising a sensor that allows an analogue signal S that is representative of the current flowing in the line 4 to be acquired, such as, for example, a current sensor 14 or a voltage sensor 12.

(14) The acquisition module is configured to obtain at least three samples S1, S2, S3 of the representative signal S, the acquisition of each signal being spaced apart by a fixed sampling time T. That is to say, as indicated in FIG. 2, if a first sample S1 is measured at a time t1, the second sample will be measured at a time t2=t1+T and the third sample will be measured at a time t3=t2+T=t1+2T.

(15) For the requirements of the invention, the sampling time T must correspond to a sampling frequency F that is more than or equal to three times the nominal frequency f.sub.0 of the signal S, thereby giving a sampling time T<=6.66 ms for a nominal frequency of 50 Hz of the electrical network 1.

(16) Preferably, in order to accelerate the detection of a fault and to be capable of detecting extremely brief transient faults, a sampling frequency F that is equal to 1800 Hz is chosen, i.e. a sampling time T of 0.556 ms.

(17) The acquisition module also comprises means, such as an analogue filter, for filtering the measurements carried out by the sensors 12 or 14, as well as sampling means allowing the signal S that is sampled to be obtained with the desired sampling time.

(18) The detection device next comprises a calculation module that receives the successive samples originating from the acquisition module. The calculation module comprises storage means for memorizing various received samples of the signal S. These storage means allow at least three samples S.sub.1, S.sub.2, S.sub.3 to be memorized, or allow samples of the signal S to be stored in a sliding manner for a longer duration.

(19) According to the invention, the calculation module determines the amplitude A of the signal S simply on the basis of the values S.sub.1, S.sub.2, S.sub.3. The sinusoidal signal S is of the form: S(t)=A.Math.sin(2.Math..Math.f.Math.t+), where A represents the amplitude, f the frequency and the angular phase. Consequently, the three successive measurements S1, S2, S3 being spaced apart by T, it may be stated that:
S.sub.1=S(t.sub.1)=S(t.sub.2T)=Asin(2.Math..Math.f.Math.(t.sub.2T)+)
S.sub.2=S(t.sub.2)=Asin(2.Math..Math.f.Math.t.sub.2+)
S.sub.3=S(t.sub.3)=S(t.sub.2+T)=Asin(2.Math..Math.f.Math.(t.sub.2+T)+)

(20) Giving:

(21) $A=\sqrt{S_2^2+{\left(\frac{S_3-S_1}{2.Math.\sin \left(2.Math..Math.f.Math.T\right)}\right)}^2}$
By means of a change of origin in order to carry out the abstraction of the angular phase and the introduction of the angular frequency =2.Math..Math.f, we get:
S.sub.1=Acos(.Math.t.sub.22.Math..Math.f.Math.T)
S.sub.2=Acos(.Math.t.sub.2)
S.sub.3=Acos(.Math.t.sub.2+2.Math..Math.f.Math.T)
The trigonometric properties of the sine function then allow the following equation to be obtained:

(22) $S_1+S_3=2.Math.S_2.Math.\cos \left(2.Math..Math.f.Math.T\right)\mathrm{where}\text{:}\cos \left(2.Math..Math.f.Math.T\right)=\frac{S_1+S_3}{2.Math.S_2}$
We therefore get:

(23) $A=\sqrt{S_2^2+{\left(\frac{S_3-S_1}{2.Math.\sin \left(\mathrm{Arc}\cos \left(\frac{S_1+S_3}{2.Math.S_2}\right)\right)}\right)}^2}\mathrm{where}\text{:}$$A^2=S_2^2+{\left(\frac{S_3-S_1}{2.Math.\sin \left(\mathrm{Arc}\cos \left(\frac{S_1+S_3}{2.Math.S_2}\right)\right)}\right)}^2$

(24) It is thereby shown that the calculation of the amplitude A depends only on three successive measured values, S1, S2, S3, without involving the frequency of the signal S or the sampling time T, without requiring previously calculated prior amplitude values to be known and without needing to mathematically model the system. The calculation method according to the invention is therefore advantageously non-recursive and non-iterative. Being independent of the sampling time T largely simplifies the calculation of the amplitude A. Moreover, when this sampling time depends on the real frequency of the signal, the sampling time must be re-calculated based on this real frequency to obtain the amplitude.

(25) Thus, by assuming that the detection device measures a fourth sample S4 at an instant t4 that is equal to t2+2T, then the calculation module will directly re-calculate a new amplitude value A based only on the samples S2, S3, S4, without using the sample S1 or previously calculated amplitude values.

(26) One of the points of interest of this method is therefore to be able to very quickly determine the amplitude of the signal S with only three samples, in contrast to approaches that generally require at least one network period in order to estimate this amplitude, and in contrast to iterative approaches that require a non-negligible time to converge towards an exact value and to refine the calculation of the amplitude. Thus, for example after restarting the electrical network following an interruption of the network, the on the fly resumption of the process is quickly achieved as soon as the three samples are measured, without having to pass through a convergence phase that generates inertia.

(27) To more quickly and simply determine the amplitude A from the signal S based on the formula above, the calculation module may favour an approach based on finite expansions of the sine and arccosine trigonometric functions, or even of the square root function, thereby avoiding the requirement for overly large calculation means. Depending on the desired precision, the chosen sampling frequency and the specifications of the microprocessor that is integrated in the calculation module, the polynomial used in the finite expansions will be of the first to the fourth order. For example, a second-order finite expansion for a sampling frequency of 1800 Hz is used.

(28) The detection device 6 next comprises a fault determination module that is capable of triggering a fault if the amplitude A calculated by the calculation module is above a first predetermined threshold M+, called the upper threshold (corresponding, for example, to an overcurrent) or if the calculated amplitude (A) is below a second predetermined threshold M, called the lower threshold (corresponding, for example, to an undervoltage, or to a flicker type effect etc.). Preferably, the thresholds are centred on the nominal amplitude A.sub.0 of the network.

(29) The upper threshold may be set on the basis of the nominal amplitude A.sub.0 to which a first margin or a first percentage of the nominal amplitude A.sub.0 is added, and the lower threshold may be set on the basis of the nominal amplitude A.sub.0 from which a second margin or a second percentage of the nominal amplitude A.sub.0 is taken. For example, standard NF EN50-160 considers a MV (Medium Voltage) voltage to be functioning normally at 10%/+10% of its nominal value and a LV (Low Voltage) voltage to be functioning normally at 15%/+10% of its nominal value; beyond these permissible bands there is a malfunction.

(30) Advantageously, the fault determination module may also memorize the type and the duration of a fault, in particular for being able to distinguish between permanent faults (not removed by the restart cycle of the network) and transient faults (removed before the end of this restart cycle).

(31) FIG. 3 gives a simplified flowchart of the method described in the invention. Upon starting a detection sequence or upon energizing the protection device, no sample S is yet memorized and an index k is initialized, for example, at the value 1. The protection method then makes provision for carrying out a measurement S.sub.k of the signal S, which measurement is filtered and digitized.

(32) While the index k is smaller than 3, meaning that three successive samples are yet to be acquired, the detection method is not capable of detecting a fault and therefore forces the calculated amplitude A to the nominal value A.sub.0, for example, of the signal S. Conversely, as soon as k is greater than or equal to 3, then the amplitude A.sub.k is calculated as being a function of the three last successive samples S.sub.k-2, S.sub.k-1, S.sub.k, according to the algorithm described above.

(33) Next, the detection method memorizes the value of the sample S.sub.k in the storage means of the detection device and incrementally increases the index k at each sampling time T before carrying out a new cycle, with the measurement of a new sample S.sub.k and a new calculation of the amplitude A.sub.k. Thus, the calculation of the amplitude is regularly updated at each sampling time T.

(34) Then, to determine the presence of a fault, the detection method passes through a determination step in which it is detected whether the calculated amplitude A.sub.k is above the first predetermined threshold M+ or whether the calculated amplitude A.sub.k is below the second predetermined threshold M in order to determine whether the amplitude A.sub.k is within a permissible band.

(35) Optionally, in order to avoid dividing by a number too close to zero, which could cause calculation errors, the detection method makes provision for carrying out a test on the value of the median sample, namely S.sub.k-1 if the three samples are S.sub.k-2, S.sub.k-1, S.sub.k (or S.sub.2 if the samples are S.sub.1, S.sub.2, S.sub.3), as the value of the median sample is used as the denominator in the proposed formula. Hence if the absolute value of the median sample S.sub.k-1 is below or equal to a minimum threshold , then the amplitude A.sub.k is not calculated but remains equal to the preceding calculated value A.sub.k-1. For example, the minimum threshold E is of the order of 0.2% of the nominal value A.sub.0 of the signal S.

(36) The graph of FIG. 4a shows, for a duration of 300 ms (scale of the abscissae x-axis in seconds), a first example of a sinusoidal signal S to be monitored as a function of time, as well as the representation of an upper fixed threshold M+ and a lower fixed threshold M. On this graph, the nominal amplitude A.sub.0 of the signal S is equal to 50, where, in this example, M+ is equal to 80 and M is equal to 40. At the beginning of the graph, it may be seen that the amplitude of the signal S is substantially equal to A.sub.0, namely the signal S oscillates between 50 et +50. Then, in the middle of the graph, the amplitude of the signal S decreases to pass below the value of the lower threshold M and, at the end of the graph, increases to pass above the value of the upper threshold M+.

(37) FIGS. 4b to 4d compare three exemplary calculations of the amplitude A of the same signal S, carried out at one and the same sampling frequency that is equal to 1800 Hz. The graph of FIG. 4b shows the result of the calculation of the amplitude according to the detection method of the invention, with second order for the finite expansions of the trigonometric functions. The graph of FIG. 4c shows a result using a method based on a fixed-window RMS (root mean square) calculation and that of FIG. 4d shows a result using a method based on a sliding-window RMS calculation.

(38) FIG. 4b clearly shows that the method of the invention gives a result more quickly than the other methods of FIGS. 4c to 4d, both for detecting a crossing of the lower threshold M and detecting a crossing of the upper threshold M+.

(39) The graphs of FIGS. 5a to 5d detail a second example that more precisely illustrates that the method according to the invention is also capable, in certain cases (in particular following the instant at which the amplitude of the signal diverges from the nominal sinusoid), of detecting an amplitude exceedence of the signal S even before this threshold exceedence actually occurs. The method thus allows the occurrence of a fault to be predicted, thereby providing multiple advantages. Specifically, this information predicting an amplitude exceedence is used by a range of equipment (e.g. protection relays, circuit breakers at the head of outgoing lines, etc.). It may also be sent as a priority to a monitoring system (GOOSE type message in IEC61850). It may thus be envisaged: to preventively open the outgoing circuit in question and hence minimize material damage and risks to personnel; to limit the imminent fault current, if the grounding of the neutral of the secondary of the transformer is a dynamically slaved Petersen coil, for example; to minimize the duration of the post-fault interruption by preparing the reconfiguration of the network after isolating the section with a fault.

(40) FIG. 5 details, for a duration of 100 ms, a sinusoidal signal S to be monitored and an upper fixed threshold M+ that is equal to 80. As for FIGS. 4b to 4d, FIGS. 5b to 5d compare three exemplary calculations of the amplitude A of the signal S of FIG. 5a, carried out at one and the same sampling frequency that is equal to 1800 Hz. FIG. 5b shows the result of the calculation of the amplitude according to the detection method of the invention, with second order for the finite expansions of the trigonometric functions. FIG. 5c shows a result using a method based on a fixed-window RMS calculation and FIG. 5d shows a result using a method based on a sliding-window RMS calculation.

(41) In FIG. 5a, it may be seen that the crossing of the upper threshold M+ by the signal S takes place at an instant denoted by T.sub.S. In FIG. 5b, it may be seen that the amplitude A calculated according to the method of the invention crosses this upper threshold M+ at an instant denoted by T.sub.A. In this example, it may actually be noted that the instant T.sub.A comes before the instant T.sub.S, by about 1.5 ms.

(42) The invention also covers an electrical unit comprising such a protection device, in such a way as to be able to interrupt the electrical network in the event of a fault. Specifically, the protection device of the invention may be implanted in various types of unit, such as, for example, a protection relay, a circuit breaker or switch or a remote monitoring/control station.

(43) In a unit, the protection device described in the invention may also potentially be combined with another device that monitors, for example, the frequency of the current flowing in the electrical network, in such a way that the decision-making takes into account the overall set of results obtained by the various devices.

(44) For monitoring a polyphase, e.g. three-phase, electrical network, an electrical unit preferably comprises a protection device for each of the three phases.