Load research for fault location in distribution feeders

C.A.Reineri and C.Alvarez

Abstract: Load behaviour needs to be considered in the algorithms for distribution systems fault locations to reduce the resulting error to practical lunits. The restrictions imposed by the load models proposed so far are discussed in the paper, and a new modelling methodology, referred to as the ’fast response model’ is proposed. New models for load elements, experimentally based and suitable for that application, are presented and aggregated at the distribution transformer level. The behaviour of the proposed models is compared with the methodologies used for that purpose, through the evaluation of their response under unbalanced abnormal voltage situations. As shown in the paper, the consideration of these models in fault location algorithms can enhance the accuracy of the location process.

1 Introduction

Errors in fault distance computation, based on voltages and currents in the sending end, in distribution feeders are usually attributed to the fault impedance and to the effect of the load supplied by the feder. Usual values for the fault resistance are - 25Q and the effect of the load is not generally considered.

The objective of response load models [I] is to predict the load behaviour under changes in the electrical supply parameters (voltage and frequency). Typical application areas for these models are demand side management [24].

The models approached in this paper fall in the category of response models and are oriented to the fault distance estimation in distribution feeders. This estimation, probably combined with other information sources (provided by cus- tomer phone calls, fault current detectors, switch positions, etc. [5]), may reduce the unavailability time forced by the fault. This requires load models with some special features: (i) the level of load aggregation is low (ii) the inputs for the model are voltage and currents before and after the fault started (iii) the model should account for the load dynamic behav- iour (iv) the voltages may suffer dramatic changes in fault con- ditions in comparison to those rated (v) the three-phase loads may have large unbalanced volt- age supply systems.

2 Fast response models

2. I Aggregation level The load is usually very distributed and the load groups have to account for diversified load elements. The more fre-

OEE, 1999 IEE Proceedmgs online no. 19990124 DO? lO.l049/ipgtdl9990124 Paper fmt d v d 28th April and in revised form 18th November 1998 C.A. Reinen is with the Facultad de Ingenieria, Univmidad de Rio WO, Rio Cuarto, Gjrdoba, Argentina C. h v a r a is with the Dept. de Ingenieria Elktrim, Univenidad Politknia de Valencia, PO Box 2012,46071 Valencia, Spah

quent load components are: air conditioners, refrigerators, lightning (incandescent, fluorescent, Hg and Na high pres- sure vapour), electronic loads, (TVs, audio equipment), water heaters, one-phase and three-phase induction motors, etc. Typical load-windows will depend very much on the specific system.

The aggregation level required to solve the fault location problem is at the ‘distribution transformer’ (medium/low voltage).

2.2 Dynamic response The fault location problem requires the solution of a circuit equation system, restricted by the border conditions imposed by the different type of faults. Such equations are presented in terms of impedance and/or admittance of the elements that form the circuit and the voltage and current values. The time requirement for the location schemes pro- posed in the literature is the time necessary for the fdter sta- bilisation, which is approximately one cycle.

The dynamic behaviour of the load may be very useful, as shown later, in the fault location process. Most of the proposed algorithms just disregard the effect of the fault duration. The methodology presented in ths paper allows the consideration of this behaviour, where the temporal analysis is restricted to the fault duration. Th~s time interval depends on several factors: fault current value, primary protection system on the area in which the fault takes place, fault type, protection co-ordination criteria, etc. Interesting results pointing out of the fault duration can be found in [6].

The electromagnetic transients during the events of inter- est are strongly damped at the distribution level, and they disappear in a few milliseconds (they never last more than a couple of cycles) [4, 61. Therefore, the dynamic aspects that should be represented in the models are more related to the load use and control rather than the electromagnetic tran- sients.

2.3 Voltage range The different types of faults force the supplied loads to a voltage that may range from a maximum value that can be even larger than the rated value to a minimum of zero volts, in addition to large voltage unbalances for three- phase loads.

115 - IEE Proc-Gener. Transm. Distrib., Vol. 146, No. 2, March 1999

2.4 Model mathematical formulation Fault location algorithms usually proceed by performing (see Appendm, Section 9) calculations based on the voltage and current applied to a circuit based on the fault charac- teristics. The instantaneous voltage and current values sam- pled at the line supply end are filtered to obtain the RMS values. This computation approach assumes that all the electrical variables are in the stationary state.

3 Review of load response models used in fault location

The application of a load model in a fault location process, for transmission lines, is presented in [7]. To determine the load behaviour in the different voltage situations, the fol- lowing formula is used:

where V , is the rated voltage, G is the conductance at the rated voltage, B is the susceptance at the rated voltage, Y is the admittance at V voltage, and np and ny are the coefi- cients for the dynamic response to that particular type of load.

It is shown in the reference that the compound effect for many loads leads to values of n,, between 1 and 1.7 and nq between 1.8 and 4.5. The authors consider that the load response coefficients can be determined from the prefault acquired data, examining the voltage changes under ‘normal’ system conditions [SI. The application scope of ths work is far different from the one presented due to the large voltage variations present during a fault. A similar approach has been presented for distribution systems in 191.

4 Elemental fast response models development

The models presented in this Section (fast response models, FRM) are based on load element behaviour obtained from the data gathered in real tests reproducing the actual fault conditions.

4.1 Experimental circuit and data measurements To obtain the experimental data for load modelling, a test lab has been built to reproduce real-life fault conditions.

Fig .I Test circuits om-phme loud

The experimental circuits for one-phase load experiments is shown in Fig. 1. The contact d can be changed along the entire resistance R for different test conditions. The load being tested should be connected to the points a and n and, in these conditions V, = Vs as the voltage drop in R can be considered neghgible.

Closing switch C1 simulates the fault. In these conditions voltage V, equals the voltage drop generated by the fault

1 I6

current circulating through d. The test is interrupted by means of C2.

The voltage and current are recorded using BMI 8020 measurement equipment [lo, 111. Typical voltage and cur- rent for one test (refrigerator load) are shown in Fig. 2.

‘T 1

0

-1

I

-2 1 Fig. 2

~ voltage current Voituge und cwwnt p.u. to U refiigerutor

4.2 Analysis methodology and results presentation To compute the model admittance parameters, the current and voltage phasors are previously obtained by using a full wave Fourier filter. A window of k samples per cycle is analysed. Should the window include signal samples pre- and postfault, the data correspond to ‘transient conditions’ and the results are not significant. According to that, the conductance and susceptance value computation is delayed a half-cycle after the fault occurrence. The voltage RMS values did not change more than 1% during the recording time (5-7 cycles).

For the three-phase motor case, a sirmlar procedure was followed. The main difference is that the conductance and susceptance need to be referred to positive and negative sequence components. Line-to-line three-phase voltages are transformed in phase to neutral components and then the sequence components are computed. From the quotient between the positive sequence current and voltage the posi- tive sequence Conductance GI and susceptance B, are calcu- lated. The same procedure is applied for the negative sequence conductance and susceptance calculations ( G2 and 8 2 ) .

5 Results and discussion

The types of elemental single-phase loads tested are: fluo- rescent lamp, Hg lamp, high-pressure Na lamp, refrigera- tor, air conditioner, microwave oven, TV and PC. A 5kW three-phase induction motor, loaded to 85‘%1 of its rated power through a stray current brake (torque proportional to the squared speed), has been tested as a prototype of the more common low voltage distribution loads.

5. I Television sets and PC The results obtained by testing a PC are shown in Fig. 3, where the G and B values are represented with respect to the applied voltage pu reduction. Tlus response is typical for equipment including a capacitor filtered rectifier source. Given a sudden voltage reduction ( 0 . 7 5 ~ ~ in this case) cur- rent circulates only in voltage peak conditions. The larger the voltage reduction is, the longer the current takes to flow.

5.2 Discharge lamps Two types of discharge lamps (Hg 250W and high pressure Na 15OW) were tested, without the power factor correction capacitors.

IEE Pro( -Gener Truritm Drmrh, Vu1 146. N o 2, Murch 1999

:::I r; 2.0

1.0 -

1.5 i ill 0.6 3

3 Q Q >- >-

I .o

0.2 0.5

0.0

-0.2 -0.5

0.25 0.45 0.65 0.85

PersomI conzputer, cotductuncc~ urui .swceptunce per unit vulues V.PU

Fig. 3 Vb,,\, = 220V and I/,,,,. = 0.38A 4- B. 1st cycle -A- B, 3rd cycle -0- B, 5th cycle ~~ 0--- B, 7th cycle 4- G, 1st cycle --A- - G, 3rd cycle -e- G, 5th cycle 4- G, 7th cycle

A progressive reduction of the conductance and sus- ceptance values with the voltage is observed. From a given voltage value the arc cannot be sustained, causing the lamp turn-off (0.8pu for Hg and HP Na lamps and - 0.55 for fluorescent ones).

5.3 Air conditioner and refrigerator The behaviour depends mainly on constructive characteris- tics of the motors as well as compressors.

The results are shown in Fig. 4. The sudden increment of the conductance (approximately five times) for 0 . 5 5 ~ ~ volt- age reduction indicates that the motor is stalled when the voltage is reduced below this voltage value. At the same time the susceptance is higher, which can be explained by the automatic connection of the starting winding.

5 -

3-

3

9 1 -

-1

-3 i 6.05- 0.30 0.55 0.80

V.PU Fig. 4 VhO,', = 220V and II,<,,,. 2: 6.3A 4- B. 1st cycle -A- B. 3rd cycle -0- B, 5th cycle -.- G, 1st cycle -A- G, 3rd cycle G, 5th cycle

Air cornlitbier, conductunce mu/ susc.eptmice per unit ~~u1c.s

5.4 Microwave oven The results are shown in Fig. 5 , where a continuous decrease of the conductance takes place when the voltage decreases. From a reduction > 0 . 7 ~ ~ the conductance behaviour can be attributed to the lights and the step-up transformer under no-load conditions. Also, the sus- ceptance has a change in sign for voltage values close to 0 . 7 ~ ~ .

IEE Pro1 -Genu Trunvn Drstrrh , Vol 146, No 2, Murch 1999

0.15 0.40 0.65 0.90

~icro ivuw oven, coruhctunce uruisusceptanceper unit vulws v,pu

Fig. 5 V,,,, = 220V and I,,< ,,', = 6.23A 4- B, 1st cycle -A- B, 3rd cycle --0- B, 5th cycle -.-- G, 1st cycle -A- G, 3rd cycle -e- G. 5th cycle

1 .o V2,PuNi P u

Fig. 6 VA,,,< = 410V and 11,,,,,, 4-- B, 1st cycle A B. 3rd cycle -0- B, 5th cycle ~~ 0-- B. 7th cycle

W G, 1st cycle -A- G, 3rd cycle -e- G , 5th cycle 0 G. 7th cycle

Motor. positive seyuenci~ co&ictunce urui susceptunce per unit vulues 10.2A

-2'5 t -5.

0.00 0.25 0.50 0.75 1 .oo V*,PU/V, ,PU

Fig. 7 Motor, rn~u t iw seqwncc. eotuhictunce r rnd swceptuncepw unit vulues C;,,,,,. = 410V and fi-~ B. 1st cycle iL B, 3rd cyclc 0 B, 5th cycle ~43 ~ B, 7th cycle

-W- G , 1st cycle A G. 3rd cycle -e- G. 5th cyclc --e-- G. 7th cycle

= 10.2A

117

5.5 Three-phase induction motor The positive and negative sequence admittance is repre- sented in Figs. 6 and 7 as a function of the relation between the negative and positive sequence voltages. Such admit- tance values were calculated from the relation between the sequence currents and voltages. The homopolar sequence component does not exist because of the lack of neutral.

6 Applications

To demonstrate the validity of the proposed models, the load elements whose models have been obtained are con- sidered in aggregations to build a distribution feeder load model. Two different load mix situations have been consid- ered: Loud mix 1: summer night: 35% air conditioner; 15% refrigerators; 25”/0 lighting (10% hp Na, 10% Hg, 50/0 fluo- rescent); 20%1 electronic load (15% TV, 5%) PC); 5% micro- wave oven. Loud mix 2: winter night: 15% refrigerator; 45% lighting (20% hp Na, 20% Hg, 5% fluorescent); 35% electronic load (25% TV, 10% PC); 5% microwave oven. The G and B values in different voltage conditions, com- puted from the lst, 3rd and 5th cycles, for load mixes 1 and 2, are shown in Figs. 8 and 9.

2’5 r

0.27 0.50 0.73 0.95 -1.5 w 0.05

Aggregated model load mix 1. conductance and susceptance V,PU

Fig. 8 -0- B, 1st cycle -A- B, 3rd cycle -0- B, 5th cycle -.- G. 1st cycle -A- G, 3rd cycle -*- G, 5th cycle

1.251 /k.,

0.27 0.50 0.73 0.95 -0.75

0.05

VPU Fig. 9 --[I B, 1st cycle -A- B, 3rd cycle -0- B, 5th cycle 4- G, 1st cycle -A- G, 3rd cycle -+- G, 5th cycle

Aggregated model load mix 2, conductance und susceptance

To show the behaviour of the proposed models in the fault location process, a 13.2kV 30km feeder with six uni-

118

formly located distribution transformers (Dy 1 1 connected) has been considered. The characteristics and layout of the feeder are those corresponding to a suburban area where the location accuracy is very important. The positive and negative sequence impedance equals 0.483 + 0 .362 iQh and the zero-sequence 0.631 + 1.588iQh. Shunt parame- ters are neglected.

Table 1:

a: Current obtained, load mix 1

Norm. cond. I,

Is

It

1st cycle Ir

Is

It

np= 1.7 Ir

Is

n,= 1.8 It

RMS (A)

6.24

6.24

6.24

2.75

2.00

4.46

3.12

2.93

5.84

Degrees

-24.7 -144.7

95.3

-67.6

-109.3

94.91

-72.65

-103.02

92.63 ~ ~~ ~

b: Current obtained, load mix

RMS (A)

Norm. cond. lr 5.9

Is 5.9 4 5.9

1st cycle Ir 2.67

Is 2.14

It 4.81

np= 1.7 lr 2.96 Is 2.77

no= 1.8 It 5.53

Degrees

-18.6

-138.6 101.4

-65.09

-71.81

111.9

-66.58

-97.21

98.63

c: Current obtained, motor

Motor Motor

Norm. cond. I,

Is It

1st cycle lr

Is

It

np= 1.7 Ir

Is n,= 1.8 I,

RMS (A)

6.96

6.96 6.96

16.85

24.22 41.04

16.43 8.94

9.60

Degrees

-15.8

-135.8 104.1

-102.5

-106.97 74.86

-59.76

148.88 93.69

Faults in distribution circuits usually imply unbalanced situations. To evaluate the proposed methodology in these conditions, a three-phase voltage system where the relation between the negative and positive components is 0.82 has been considered (this correspond to the voltage in the last distribution transformer in the described test feeder after a phase-phase-ground fault at the end). Tables 1u-c show the currents on the medium voltage side for aggregation mixes I , 2 and a load formed by three-phase induction motors. The currents obtained by using eqn. 1 are also included, as well as the balanced currents previous to the fault.

It can be concluded, from these results, that the load cur- rents during the fault may be very important compared to the normal conditions of load currents and that there is an important disparity between the model values from the

IEE Proc-Gener. Trunsm. Distrib. , Vol. 146, No. 2. March 1999

proposed methodology and the ones obtained of the appli- cation of eqn. 1.

The estimated fault (single phase to ground) for different distances and fault resistances are shown in [2]. Load mix 1 has been considered and three dlfferent alternatives have been used to account for the load behaviour: the proposed models, eqn. 1, with np = 1.4 and n4 = 3.3, and no model (Y = 0). Two load conditions are considered for every test: load 1, which corresponds to a prefault load producing a 60/0 voltage drop in the feeder, and load 2 with a 12% drop. Voltage and currents in the sending end are computed by means of a fault load flow. The location algorithm used is based on a similar concept as those used in [I, and it is restated in the Appendix (Section 9). Voltage and currents in the sending end are computed using a faulted load flow.

The miscalculation of np and nq parameters while using the model defined by eqn. 1 may lead, as shown in Table 2b, to large errors in the distance estimation.

Table 2:

a: Results obtained with bath models and with zero load admittance under different line to ground fault conditions

Distance Rf Load type Model, krn eqn. 1, krn Y = 0, krn

10km 2 8 Load1 10.00 10.03 9.88 Load 2 10.00 10.02 9.84

158 Load 1 10.07 10.36 10.03 Load2 10.06 11.10 10.52

20krn 2 8 Load 1 19.99 20.10 19.32

Load2 19.99 19.92 19.01 158 Load 1 20.08 20.92 19.12

Load2 20.08 21.19 18.98

30km 2 8 Load 1 29.98 30.95 27.89

Load2 29.98 29.58 26.93

158 Load1 30.10 31.34 27.12 Load 2 30.10 31.05 26.28

b: Results obtained for line to ground fault D= 30 km

np nq Dtoload 1 Dtoload2

1.2 1.5 30.59 30.24 3.3 31.92 31.92

4.5 32.31 32.29

1.4 1.5 30.04 29.43

3.3 31.34 31.04

4.5 31.73 31.41

1.8 1.5 29.13 28.18

3.3 30.38 29.7 1

4.5 30.76 30.06 Rf= 158 with different n,and nq

It can be concluded, from this paragraph, that the loca- tion process is improved with the use of the proposed mod- els. Thls is more important for hgher load levels, where the use of the proposed methodology may be necessary to get reliable results.

7 Conclusions

The need for new models for fault location in distribution feeders has been pointed out, and a new methodology for that purpose has been proposed. The proposed type of load model, the fast response model, has been characterised and defined. The proposed modelling methodology is

IEE Proc.-Gener. Transm. Distrib., Vol. 146, No. 2, March 1999

based on extensive appliance tests, and some experimental results for elemental load models are presented. The suita- bility of the proposed models in the field of fault location in distribution systems is shown by means of simulations based on the experimental results. The need for more accu- rate models for this application is also quantified by means of simulation. The elemental models have been aggregated at the distribution transformer level. For a fault condition imposed voltage profde the results obtained by the classical model and the ones for the model proposed here were com- pared. On ths comparison, substantial differences were observed, which will undoubtedly produce considerable errors in distance estimation.

8

1

2

3

4

5

6

7

8

9

10

I 1

9

References

ALVAREZ, C., and ARO, S.: ‘Stochastic load modeling for energy distribution applications’, TOP, Revista de la Sociedad Espariola de Estadktica e Investigacidn Operativa, 1994, 2, (I), pp. 151-166 LVAREZ, C., and GABALDON, A.: ‘Integrated electricity resource planning’ in ‘Distribution load modeling for demand side management and end-use efficiency’ (Kluwer Academic Press, 1994) LVAREZ, C., MALHAME, R.P., and GABALDON, A.: ‘A class of models for load management application and evaluation revisited‘, IEEE Trans. Power Syst., 1992,7, (4), pp. 1435-1443 CHEN. M.S.: ‘Determinim load characteristics for transient mrform- ances’. Prepared by the UAversity of Texas at Arlington, Enirgy Sys- tem Research Centre, June 1978, EPRI EL 849-3 YPSILANTIS, J., YEE, H., and TEO, C.: ‘Adaptive, rule based fault diagnostician for Dower distribution networks’, IEE Proc.-C, 1992, 13% (6), pp. 461468 BURKE, J.J., DOUGLASS, D.A., and LAWRENCE, D.J.: ‘Distri- bution fault current analysis’. Prepared by Electric Power Research Institute, Palo Alto, CA, May 1983, EPRI EL-3085 SRINIVASAN, K., and ST.JACQUES, A.: ‘A new fault location algorithm for radial transmission lines with loads’, IEEE Trans. Power Deliv., 1989, 4, (3), pp. 16761682 SRINIVASAN, K., NGWEN, C.T., ROBICHAUD, Y., ST.JACQUES, A., and ROGERS, G.J.: ‘Load response coeffcients monitoring system: theory and field experience’, IEEE Trans Power Appur. Syst., 1981, PAS-100, (8), pp. 3818-3827 ZHU, J., LUBKERMAN, D.L., and GIRGIS, A.A.: ‘Automated fault location and diagnosis on electric power distribution feeders’, {EEE Trans. Power Deliv., 1997, 12, (2), pp. 801-809 8020 PQNodePlus User’s Guide’. Basic Measuring Instruments

PQNode Application System Software (PASS) User’s Guide’. Basic Measuring Instruments (BMI)

p w

Appendix

To compute the distance to the fault from the sending end of a feeder (see Fig. IO), the following equations should be used:

I Lk+l : d l I

n

I Fig. 10 Typical &tr27utwn$ea’er with lo&

phase to ground:

phase to phase: V , + V n + K = L I p + I n + I z ] Z f (2)

(3) v, - v, = LIP - I,JZf

119

three-phase:

V and I being the components (positive, negative and zero) of the voltage and current in the fault. Zf is the fault impedance, usually considered as resistive, which results in

v, = I,Zf (4)

( 5 )

voltage in a generic feeder load bus m can be easily com- puted:

v, = V,-l - ZLmItm (6) and

Itm Itm-I - I cm-~ (7) where 2 is the line impedance (by length unit), L,, and It, are section m length and current, and Icln-l the load current in bus m - 1, computed as

Icm 1 = Vm-lym-1 ( 8 )

(9)

Y,, being the load admittance. Obviously,

Ipos = IpTe - IF

and k + 1 voltage is

vk+1 = v k - Zd1p-e - z (Lk+1 - d)Ipos (10) The fault voltage is

V F = v k - ZdIpT, (11) The location process can be solved in an iterative way, which requires initial values of D and ZpOs. From that and by using eqns. 4 6 Iprc, can be computed as

Vk+l can be approximated by IpTe = Itk - Ick

v k + l = v k - ZLk+lIpTe

(12)

(13) To compute Icnl from k + 1 to the end, a flat voltage profile with all voltages equal to Vk+, can be initially considered. Then, I,,,, IF (eqn. 9), VF (eqn. 11) and finally d (eqn. 5 ) can be actualised.

The process is completed by actualising all node voltages from k + 1 to the end (eqn. 10) and computing a new d. Convergence is achieved when similar d values are obtained in two consecutive iterations. Good convergence has been observed in all tested situations.

I20 IEE P r w - G m e r . Transm. Disfrib.. Vol. 146. No. 2, March 1999