Down-conductor fault detection and location via a voltage based method for radial distribution networks ! L. Garc!ıa-Santander, P. Bastard, M. Petit, I. Gal, E. Lopez and H. Opazo Abstract: A method to detect and locate a high-impedance fault caused by a downed conductor on a distribution primary feeder cone is presented. The protection system in radial networks generally consists in placing current detectors along each feeder lenz. A down-conductor fault typically has a very low current values which is often not detectable by conventional overcurrent protection devices. The proposed method is not based on current measurement but instead on voltage measurements performed at MV/LV stations. This method may be particularly interesting if LV potential transformers are already present in the network. The method is divided into two steps. The first step consists in determining whether each sensor is above or below the fault. The second step consists in determining the fault section, from a purely topological method. EMTP simulations are presented to highlight the effectiveness of the proposed technique.
1
Introduction
A high-impedance fault (HIF) characteristically has a very low current value which is often not detectable using conventional overcurrent protection devices. The most frequent and worrying type of HIF occurs when an energised primary conductor breaks and falls to earth. This situation is especially dangerous since there is a risk that of a member of the general public receiving electric shock and there is also a fire hazard. Therefore, from both public safety and operational reliability viewpoints, the detection of HIFs is critically important. The most common detection schemes for HIF are based on the adjustment of overcurrent protection devices. Since the electric current level associated with a HIF is not significant enough for it to be distinguished from other events, this design has lead to several unexpected service interruptions [1]. It is generally accepted that the currently used protection equipment does not detect about 30–50% of downed conductor faults. This is the reason why a considerable amount of work has been performed in recent years to improve the detection of this kind of fault. It is clear that downed conductor faults are not the only way to produce a HIF: however, but it is the most probable cause. The electrical characteristics of the HIF (high-order harmonics and small short-circuit currents), together with the random behaviour of the occurrence of these faults, r IEE, 2005 IEE Proceedings online no. 20041300 doi:10.1049/ip-gtd:20041300 Paper first received 11th December 2003 and in revised form 7th September 2004 L. Garc!ıa-Santander, P. Bastard and M. Petit are with SUPELEC, France I. Gal is with Schneider Electric, France L. Garc!ıa-Santander, E. L!opez and H. Opazo are with the Department of Electrical Engineering, University of Concepci!on, Casilla, 160.c, Correr 3, Concepcion, Chile E-mail:
[email protected]
180
increase the difficulty of their detection by the protection equipment. In alternative techniques have been repeated for example a spectral analysis of currents to detect the HIF is utilised in [2–13]. The unbalanced voltages produced by the fault at the end of the transmission lines are used to detect and localise the HIF in [14]. Neural networks are used in the detection of a HIF in [15–17]. In addition, in [18] and [19] an algorithm based on a residual currents analysis (DESIR) is applied to achieve the selective detection of a HIF. Also, since the detection of the faulty currents and the interruption of the affected circuits are made at substations serious effects on the efficiency of these systems occur. A first limitation lies in the lack of selectivity. If a potential HIF is detected by a substation-based protection system, then the entire feeder is de-energised. Then there are two actions to that can be followed: (i) the immediate reconnection of the faulted branch, which implies a risk of damage to people and equipment if the fault really exists; and (ii) send line engineers to verify the state of the lines before making the reconnection, which implies a long service interruption. These points highlight the importance of developing HIF identification techniques. A second limitation of a substation-based current monitoring system is the low magnitude of the current produced by the HIF (dry asphalt or sand: 0 A; wet sand: 15 A; dry sod: 20 A) [1]. Several types of HIF such as a conductor falling on to a paved road, do not generated any measurable current. Any current monitoring system would find it difficult to detect these kinds of occurrences. Most of the limitations above may be eliminated if the protection system (fault detection and clearing) is distributed along a feeder line and the monitored parameter is the voltage inbalance instead of the current. The method we intend to propose at is not based on current measurements but instead on voltage measurements located at MV/LV stations. This method may be particularly interesting if LV potential transformers are already available in the distribution system. The method is divided into two steps. The first step consists in determining whether each sensor is above or IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 2, March 2005
below the fault. The second step consists in determining the fault section using a purely topological method. This method results in the reliable detection and location of a broken conductor.
This topological method has two important advantages: (i) it leads in a sure way to the faulty section; and (ii) it is systematic, and easily programmable. 3
2
Determination of the active sensors
Topological method
Let us assume that a downed conductor fault has occurred somewhere on a radial distribution feeder connected to a HV/MV station. Let us also suppose that each end of this feeder is equipped with a sensor that posses an algorithm to analysis date and determine if it is above or below the fault. A sensor located below the fault will be called an ‘active sensor’. If all the active sensors on a given feeder are known, it is possible to determine in a systematic manner the faulty section, by applying the following algorithm.
The ‘active’ sensors method (i.e. the sensors are located below the fault) is described in [20]. The method to determine the active sensors is always based on voltage measurements. In a positive sequence, all the points located below the fault should see the same voltage drop.
3.1
Positive sequence voltage
The positive sequence differential voltage can be defined as follows: DVd ¼ jVd Vdo j ð1Þ
Step1: Number the nodes. Step 2: Make a table of all the branches, each branch being described by two nodes (a downstream node and an upstream node). Step 3: Locate all the ‘active’ sensors. Step 4: For each active sensor, list the nodes between the sensor and the HV/MV substation. Step 5: Find the nodes common to all the roues between active sensors and the HV/MV station. Step 6: The fault is between the last two downstream nodes located on the common way.
where Vd represents the positive sequence voltage after the fault and Vdo represents the positive sequence voltage before the fault. It is also possible to define a positive sequence relative differential voltage: ð2Þ DVd;pu ¼ jVd Vdo j=Vdo
Let us apply this method to the simple network of Fig. 1. The active sensors (as found by the proposed algorithm) are at nodes 3, 4 and 5. From Table 1, it is easy to determine the routes (downstream to upstream) between each active sensor and the substation. They are: (i) 3–10–9– 8–7–6; (ii) 4–9–8–7–6; (iii) 5–10–9–8–7 The common way is 9–8–7–6. The last two nodes are 9 and 8. Therefore, the fault is between nodes 9 and 8.
4.1
HV MV 6
7 8
1
2 F
The relative differential voltage has the advantage of remaining constant below the fault, even if the positive impedance of the source is not small when compared with the impedance of the loads. 4
Simulations
Simulated network
The simulated network is described in Fig. 2. and Table 2. The network can be entirely aerial. The characteristics of the overhead lines are then: Zero sequence: A resistance ro of 0.9 O/km, reactance xo of 1.2 O/km, and capacitance co of 0.01 mF/km. Positive sequence: resistance r1 of 0.3 O/km, reactance x1 of 0.4 O/km, and capacitance c1 of 0.02 mF/km. The MV/LV transformers are simulated from typical data, as described in the NF C 52-112-1 standard. Simulations were performed for three values of the fault resistance: 1 O, 100 O and 1 kO, each value representing the impedance of the contact between the grounded conductor and the soil. There are two ways for the downed conductor to come into contact with the soil: (i) the source line is grounded (Fig. 3a) or (ii) the load line is grounded (Fig. 3b). For every simulated case, two figures are plotted, showing the sequence at the increasing relative differential voltages for the grounded source has and the grounded load line faults. On each figure the number of the node where the quantity is estimated is directly written on to the curve itself. The x-axis label therefore has no meaning.
3 9
4.2 10 5
4 connection voltage sensor
Fig. 1
Case 1
This case consist of on overhead network, a asistive earthings of 40 O, a broken conductor and a resistance fault of 1000 O. The fault is an open-circuit fault, as created by a broken conductor. It is in the middle of the section 9– 10. The platform sequence of the increasing relative differential voltages (see Fig. 4) leads to a very easy detection of
Fault in a radial distribution network
Table 1: Sections of the network Downstream Upstream
3
5
10
4
9
2
8
1
7
10
10
9
9
8
8
7
7
6
IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 2, March 2005
181
source 63 kV P cc = 100 MVA
source
transformer 63/21 kV
load
ZN
Rdéf
0 transformer 20 kV/ 400 V 630 kVA ; ucc = 4% 1 3
a
load : 0 < P< 500 kW cos = 0.9
19 2
4 28
8
source
7 20
27
11 5
F 9
10
6
12
b
13
Fig. 3
15 21
26
18 14
17
24
MV feeder end
25 22
0.7
23
voltage sensor
0.6
The simulated network
Sensor
0–1 19
1–3 3–4 3–5 5–6 5–7 7–8
20
21
28
22
23
19 1
2
3
4
5
6
8
9
10
25
27
26
24
8
9
10
a 0.50 0.45
5
9–15
2
15–16
23
0.1
15–17
22
3 1
0.40 0.35 0.30 0.25 0.20 0.15
27
1 0.3
0.10
12–13
26
1
0.05 19
12–14
25
1
0
12–18
24
2
10–12
7
x label : no meaning
2
2
9–10
26
0
4
7–9
10–11
0.1
1
3 28
27
0.2
6 21
25
0.3
2.5 20
24 0.4
3
differential voltage
1–2
Length, km
differential voltage
0.5
Table 2: The lengths of the sections Section
The downed conductor
a The source line is grounded b The load line is grounded
16
node
Fig. 2
load
Rdéf
1
20 2
23
21
28
22
3
4 5 6 7 x label : no meaning b
the sensors located below the fault, i.e. sensors 24, 25, 26 and 27. Let us apply the algorithm to determine the faulty section: 24 12 10 9 7 5 3 1 0 25 12 10 9 7 5 3 1 0 26 12 10 9 7 5 3 1 0 27 10 9 7 5 3 1 0 The common way is 10–9–7–5–3–1–0 The last two nodes are 10 and 9. So the fault is between 10 and 9. 182
Fig. 4
Overhead network, Rf ¼ 1 kO, broken cable
a The fault is a grounded source line b The fault is a grounded load line
4.3
Case 2
This case consists of an overhead network, a resistive earthing of 40 O, a broken conductor and a resistance fault at 100 O. Again plotting sequence of the increasing fault relative differential voltages (Fig. 5) leads to a very easy IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 2, March 2005
0.7
0.7
0.6
0.6 0.5
27
24
25
differential voltage
differential voltage
0.5 26
0.4 0.3 0.2 0.1 19
20
1
2
21
28
23
27
8
9
10
26
27
24
8
9
10
24
0.2 19
22
23
21
28
3
4 5 6 7 x label : no meaning
20
22 0
3
4 5 6 7 x label : no meaning a
8
9
1
10
2
a
0.40
0.35
0.35
27
25
24
26
25
0.30
0.30 differential voltage
0.25
0.25 0.20 0.15 0.10 0.05 19
20
21
28
22
0.20 0.15 0.10 0.05 19
23
22
21
28
3
4 5 6 7 x label :no meaning
23 20
0
0 1
Fig. 5
26
0.3
0.1
0
differential voltage
25 0.4
2
3
4 5 6 7 x label : no meaning b
8
9
1
10
Overhead network, Rf ¼ 100 O, broken cable
2
b
Fig. 6
Overhead network, Rf ¼ 1O, broken cable
a The fault is a grounded source line b The fault is a grounded load line
a The fault is a line is grounded source line b The fault is a grounded load line
detection of the sensors located below the fault. The sensors are 24, 25, 26 and 27 and the fault is between notes 10 and 9.
lower than 70% of its nominal value, an alert signal can be sent to the control station to indicate a downed conductor above the sensor.
4.4
6
Case 3
This case consists of an overhead network, a resistive earthing of 40 O, a broken conductor and a resistance fault of 1 O. Again plotting the sequence of the increasing relative differential voltage (see Fig. 6) leads to a very easy detection of the sensors located below the fault. The fault again is located between nodes 10 and 9. 5
Discussions
Figure 4 shows that the relative differential voltage (DVd) increases by between 40–50% for the dowstream sensors, when the conductor breaks. It is then possible to calibrate a sensor with a critical threshold where (i) under this threshold the sensor is considered to be above the fault and above the threshold the sensor is below the fault. In any simulation (1 O, 100 O, 1 kO) the relative differential voltage, does not depend on the fault resistance for a grounded source line (DVd ¼ 50%). However, DVd increases with the fault resistance for a grounded load line (DVd ¼ 35–47%). The small relative differential voltage is 35% for a 1 O fault resistance and a grounded load line fault. These results show that a downed conductor induces a positive sequence voltage drop greater than 35%. Therefore, when a sensor measures a voltage IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 2, March 2005
Conclusions
A topological method has been described that is systematic and easily programmable. The reference level to determine these sensors located below the fault (‘active sensors’) is easy to calibrate. A pertinent quantity has been defined, from voltage measurements, to determine which sensors are ‘active’. The method requires a voltage sensor to be placed at each end of a MV branch and the calculation of a characteristic quantity which is transmitted to a fault central point for a comparative analysis. This method offers an efficient route to locate a fault created by a downed conductor fault. The proposed fault location algorithm has two key advantages. Firstly, it can be easily used in networks that already have voltage sensors installed in the MV/LV stations, for instance to control the voltage available on the LV network. Secondly, it is very efficient at locations open-circuit faults (less a broken cable). In such a case, the measurement precision is no lower a problem and the faulty section can be determined very easily. It is possible to decrease the number of sensors required to detect a fault if it occur in a given zone including several sections (for example between two switchgears), and not between two adjacent nodes. 183
7
References
1 IEEE Tutorial Course 90EH0310-3-PWR:‘Detection of Downed Conductors on Utility Distribution Systems’ (IEEE, Prsestoways, NJ, USA) 2 Jeering, D.I., and Linders, J.R.: ‘A practical protective relay for downconductor faults’, IEEE Trans. Power Deliv., 1991, 6, (2), pp. 565–574 3 Benner, C.L., and Russel, B.D.: ‘Practical high-impedance fault detection on distribution feeders’, IEEE Trans. Ind. Appl., 1997, 33, (3), pp. 635–640 4 Aucoin, B.M., and Russell, B.D.: ‘Detection of distribution high impedance faults using burst noise signals near 60 Hz’, IEEE Trans. Power Del., 1987, 2, (2), pp. 342–348 5 Aucoin, B.M., and Russell, B.D.: ‘Distribution high impedance fault detection utilizing high frequency current components’, IEEE Trans. Power App. Sys., 1982, 101, (6), pp. 1596–1606 6 Lee, R.E., and Bishop, M.T.: ‘A comparison of measured high impedance fault data to digital computer modelling results’, IEEE Trans. Power App. Syst., 1985, 104, (10), pp. 2754–2758 7 Calhoun, H., Bishop, M.T., Eichler, C.H., and Lee, R.E.: ‘Development and testing of an electro-mechanical relay to detect fallen distribution conductors’, IEEE Trans. Power Appar. Syst., 1982, pp. 1643–1650 8 Kilar, L.A., Rosado, M., Farnsler, H.F., and Lee, R.E.: ‘Innovative relay methods for detecting high impedance faults on distribution circuits’. Presented at the 1979 American Power Conference 9 Sultan, A.F., Swfit, G.W., and Fedirchuk, D.J.: ‘Detecting arcing downed-wires using fault current flicker and half-cycle asymetry’, IEEE Trans. Power Deliv., 1994, 9, (1), pp. 461–470 10 Emanuel, A.E., Cyganski, D., Orr, J.A., Shiller, S., and Gulachenski, E.M.: ‘High impedance fault arcing on sandy soil in 15 kV distribution feeders: contributions to the evaluation of the low frequency spectrum’, IEEE Trans. Power Deliv., 1990, 5, (2), pp. 676–686
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