480 IEEE Transactions on Power Delivery, Vol. 9, No. 1, January 1994

ADAPTIVE RELAY SElTING FOR STAND-ALONE DIGITAL DISTANCE PROTECTION

Y. Q . Xia K. K. Li A. K. David Tianjin University, China

Abstract: Reach accuracy of a distance relay on transmission lines is adversely affected by fault resistance combined with remote-end infeed which is not measurable at the relaying point. Different network conditions correspond to different remote-end infeed behaviour and in conventional setting a safety margin is necessary so as to avoid maloperation. In this paper an adaptive setting concept which can overcome this disadvantage is proposed. A microprocessor based distance relay with this new technique can respond to network conditions that change from time to time and computer simulation has confirmed the validity of this new concept.

INTRODUCTION

Digital relaying has been developed for more than twenty years since Rockefeller conceptualized a single computer performing all the relaying functions in a substation. Recently, adaptive transmission relaying concepts [ 1,2] became an attractive possibility due to the rapid development of computer and coininunication technologies. Relatively inexpensive computers for substation control, data acquisition and on-line analysis are now available while on-going development of large bandwidth communication systems, using new technologies such as fibre- optics, are linking together all modes of utility operation.

In keeping with this trend adaptive protection achieves better performance from a protection system by allowing the settings to be made with consideration for fewer contingencies than is presently the case. Fewer constraints on a problem leads, in general, to a solution which is closer to the ideal. Sophisticated digital relays can be designed to adapt to varying conditions, that is, be free from the limitations imposed by the need to treat changing network conditions as unknown constraints.

Digital distance relays using microprocessor technology have indeed overcome some of the traditional boundaries in protective relaying. However, even with digital distance relaying, the practice has been to design the scheme on the basis of fixed relay settings. Changes would only be made when the configuration or system was significantly modified. Advantages of adaptive distance protection have been discussed and a practical scheme of adjusting the zone-1 boundary has been proposed [3]. Improved performance was obtained by using

93 WM 038-0 PWRD by the IEEE Power System Relaying Committee of the IEEE Power Engineering Society for presentation at the IEEE/PES 1993 Winter Meeting, Columbus, OH, January 31 - February 5, 1993. Manuscript submitted January 16, 1992; made available for printing December 28. 1992.

A paper recommended and approved

Hong Kong Polytechnic, Hong Kong

an algorithm to adjust the boundary angle. However, if the system conditions vary in a wide range and faults occur through high arc resistances the relay may lose selectivity.

This paper presents an new adaptive scheme for digital distance relay setting motivated by the desire to respond to the network conditions automatically. The relay can then operate faster and be more sensitive to various faults under different conditions without loosing selectivity.

This scheme is intended for use with today’s microprocessor- based commercial design philosophy. The scope for widespread acceptance by utility relay engineers is taken into account. Not only is it from a hardware viewpoint, much like existing hard- wired designs, but it is also capable of using available facilities in local substations. The proposed scheme requires only slow- speed-responses such as those of a substation control and data acquisition (SCADA) system, in contrast to the high-speed channels required by present carrier protection schemes. Equivalent parameters of the external system required for the adaptive setting can be developed at the substation using a locally available system impedance data base and circuit-out information from local events and from remote computers.

ADVERSE EFFECT OF FAULT RESISTANCE

A digital distance relay uses sampled voltage and current data from the relaying point for measuring the apparent impedance and then uses an appropriate characteristic to make proper decisions to disconnect a faulted line. With reference to Fig. 1, suppose a distance relay is installed at M and a fault occurs at F through a fault resistance R,, the apparent impedance of phase-A measured by the relay can be expressed as

In the above conventional reactance-type measurement using the a.c. quantities available at M only it is impossible to determine the infeed current IAN flowing through the fault resistance. The remote-end infeed is dependent not only on fault location and fault resistance but also on source impedance of the two ends. Consequently, in conventional distance relay setting, including digital distance relaying practice, a safety margin is used in zone 1 to avoid maloperation. A certain percentage of the line, therefore, falls into zone 2. Reducing this margin to a minimum and covering almost the whole line in zone 1 is beneficial for system security, and if high speed data communication is available for information exchange with the remote end during faults, this will not be a problem. The problem is, if no such high-speed channel is available how can a stand-alone distance

0885-8977/94/$04.00 0 1993 IEEE

481

relay be made responsive to "existing" system conditions? Searching for a solution to this problem, and exploiting the fact that these "existing" conditions are known prior to a fault from available substation control and data acquisition system information, is the objective of this paper.

ADAPTIVE SETTING mMDAMENTALS

System conditions external to the protected line influence relay performance; to demonstrate this effect and outline the adaptive relay setting scheme a two-terminal model with the external system represented by an equivalent source potential and source impedance is shown in Fig. 1. In the following analysis a single- line-ground fault at F through a fault resistance R, is examined and a digital distance relay installed at M is considered.

-

Figure 1 Phase-A to ground fault model E, and EM are the equivalent potentials at the two ends; EM / E, = h e-j*, h is the amplitude ratio and, 6 is the power transfer angle; ItD is the pre-fault load current in the line, ah(, and IMP are the fault currents derived in the Appendix and I,, IAN are the line currents under fault conditions obtained by superposition; Z,, and Z, are the equivalent source impedances; Z,, is the line impedance from M to F and, ZLN is the line impedance from N to F.

Defining

'1, = zlSM + zlLM ; '1, = '1SN + '1LN ;

'OM = 'OSM + 'OLM

'ON = 'OS, + 'OLN (2)

where, subscription "1" and "0" represent positive- and zero- sequence components respectively, the following relation between the apparent impedance Z, measured at M, and the actual impedance ZILM from M to F is also derived in the Appendix.

(3) (zc +3RF)K, + 2C1 + c0<1+3&)

Z" = z,, +

where,

(4)

and, C, and CO are positive- and zero-sequence distribution factors respectively and I<oL the zero sequence compensation factor, all defined in the Appendix.

From equation (3), it is obvious that if R, = 0, the apparent impedance will be the actual line positive sequence impedance from M to F no matter how the system configuration be changed. If R, is not equal to zero, the measured impedance will be affected by system configuration and operating conditions. Using the definition

it can be seen that if AZ is capacitive the measured reactance will be less than the actual value. On the other hand, the measured reactance will be larger than the actual value if AZ is inductive. In general, either case is possible and, therefore, the relay may overreach or underreach under different system conditions. In traditional distance relay setting the most serious overreach situation would be taken into consideration to prevent false tripping and, therefore, a certain length of the protected line would be left outside zone 1. Electrical power engineers have dreamed for many years of setting the distance relay to respond to faults in the whole line immediately. An adaptive setting scheme can almost make this true.

The positive- and zero- sequence impedances of transmission lines can be calculated off line and, except for double lines on a same tower, they are generally considered fixed. It can then be seen from equation (6) that the impedance drop resulting from remote-end infeed is dependent on not only fault location and the fault resistance, which are not known before a fault, but also on six network configuration and operation parameters,

The lumped external impedance Z,, and Z,, can be updated from local circuit impedance, breaker status information and on- line measurements of system load. Local computers make this feasible and linking to the remote computers provides more accurate updating. More about the development of the external impedance model and its updating have been discussed in reference [l]. The potential equivalents h and 6 can be also obtained and updated from stored network data and on-line measurements. Before describing an adaptive relay setting scheme which can respond to these system changes, computer simulation results of how network conditions affect the measured impedance are presented. Splitting equation (3) into real and imaginary parts, that is measured resistance RA and measured reactance X,, the simulation results for different values of Rp and fault location are given in Fig.2.

The most interesting case for distance-relay-setting study corresponds to faults located near the remote end and, therefore, numerous computer simulations for a fault located at 95% of line length on a typical 128km, 400kV overhead line have been done. One parameter is changed in each case while the others remain fixed. The results confirm that the measured reactance and resistance and the relay-reach trail in R-X plane are affected by changing any one of the parameters. Fig.3 shows just one such result, the case of changing the source ratio amplitude h.

482

Measured Resistance

-200

-300

I ,

\

0 40 80 120 160 200

0 4 0 80 120 160 200 0 0

2 -100

-200

-300

RP@)

Figure 2 - (a)

- R ; l o o s <' y,\- - \ - - \

Measured Reactance

40 F

0 40 BO 120 160 200

Rp(n)

-101

Figure 2 - (b)

R-X Plane I

.

40 BO 120 160 200 -10'

RA(0)

Figure 2 - (c)

Figure 2 Variation of measured impedance with fault resistance for different fault locations:

60%( - - - --), SO%( - -), 100%( - - -), under system conditions given by: Z1,=20L85"R, ZM/Z,== 1.5, Z,,= lOL85'R, L I Z l m = 1.5, h =0.95, 6 =20°, Z1,=36.8L86"R, Z,, = 1 1 1.8L83 "0.

0% (-), 20%( ), 40% ( - - ),

Measured Resistance

R A N

Figure 3 - (a)

Measured Reactance

300 I 200

100 - c - 0 '> 1 x4

-100

Figure 3 - (c)

Figure 3 Variation of measured impedance with fault resistance for different potential ratios (h):

l . O l ( - - - - ) , 1.02(- -), 1 . 0 3 ( - - -), under system conditions given by: Z1,=20L850R, Zm/Zlw= 1.5, Z,sN= lOL85'R, L / Z l , = 1 . 5 , 6 =20°, Z1,=36.8L86"R, Z,,= 1 1 1.8L83 " R , fault location =95 %.

0.98 ( ~ ), 0.99 ( 1, 1 . 0 0 ( - - 1,

483

setting precision. Also, the nonlinear boundaries will make it more complex during the on-line indexing. The latter approach will be a better solution if the on-line computation is within the processing capability of commonly used 'micro-processors and this method is described further below.

ADAPTIVE SETTING SCHEME

1. An Ideal TriD Characteristic:

If system conditions @, 6, Z,, Z,) are fixed and Rp and fault location varied, four boundary lines, defined below, are obtained by computer simulation.

Line I: solid faults at different locations; Line 11: faults at a relay-reach end (95% of line length) with

different fault resistance up to 20033 Line III: faults at different points with a 2000 fault resistance; Line IV: faults at the relaying point with different fault

resistance up to 2000;

The four lines and the included area constitute what may be designated an ideal trip region under the prevailing system conditions.

R-X Plane R,=OSZ

Y

ideal tr ip region

I

40 80 120 160 200 R A ( ~ )

2. On-Line Adaptive Avvroach

The on-line computation of the adaptive setting scheme consists of two parts, viz. (i) the computation of the setting boundaries, (ii) the calculations performed during a fault. This scheme has been carefully designed in order to minimize on-lime calculation of both parts while m a i n d n g high reach accuracy.

A number of straight line segments are used to approximate the curved boundaries in Fig.4. The number of the straight lines is determined by the precision required and processing capacity of the target microprocessor. In this study case, three straight limes (2, 4, 6) are used for line 11, two (8, 10) for line III, two (12, 13) for line IV and line 1 for line I -- see Fig.5.

a

Figure 5 Adaptive setting characteristic and topological trip-decision logic

Figure 4 An ideal trip region under given system conditions: Z,,,=20 f 85"Q, Zo,,/Z,,M= 1.5, Z I S N = 10 f 85"0 , ZosN/Zl,= 1.5, h=0.95, 6=20".

As shown in the figures in the previous section, system configuration and its operating conditions affect the measured reactance and resistance; that is these four lines and the included area vary with system conditions. Therefore, how to make a distance relay operate sensitively as these conditions vary still remains a problem. To solve this problem, two different approaches can be proposed at this stage. Firstly, setting patterns under all possible system conditions can be calculated off-line and stored in a data table for indexing during faults. Alternatively, only one set of setting parameters are calculated on-line and these parameters are renewed from time to time as the system conditions change beyond a certain limit. When a fault occurs the relay will operate with the latest setting parameters, an optimal trip boundary. The former approach, a common table-indexing method in computer application, may be faster in on-line processing due to less on-line computation. However, storing all possible setting patterns would require a huge table to cover all the system parameter combinations. A large amount of computer memory would be needed to maintain

Each line divides the R-X plane into two and we suppose that a "line output" is positive if the measured impedance falls on one side of the line and negative on the other side. If the ideal trip region remained a convex shape these linear outputs and some "AND" operations could well determine a trip decision during fault. However, since different system conditions will give rise to different curved boundaries the decision region may be non- convex. Hence mere logical "AND" operations of the linear outputs cannot be relied upon for the trip decision. For this reason additional lines 3, 5, 7, 9, 11 are used to divide the decision region into several sub-regions. The linear outputs for straight lines 1 through 13 in F i g 5 correspond to signals L1 through L13 in Fig.6. A combination of logical "AND" and "OR" operations of these signals can now be used to obtain a reliable trip decision.

This topological trip-decision logic is described with reference to Fig.6 in the following paragraphs. Each linear output, L1 for example, is connected to the measured reactance and resistance through two weights, W,, and W,z. A bias weight W,,, is also connected to the linear output. The output value is defined by

Q

484

I Xa

1L1 t L 2 - ' 1 L13 Figure 6 Calculation architecture in adaptive setting

Positive and negative values of the output determine the two sides of the line. A critical thresholding condition occurs when the linear output equals zero:

therefore,

(9)

It is apparent that the above equation expresses a straight line having slope and intercept given by (-WI2Tw,,) and (-WloTwlI) respectively and, therefore, the three weights, W,,, W,, and WIZ, determine the slope, intercept, and side of the separating line. In the case of line " 1" in Fig.5 the intercept is zero and the slope can be determined by the resistance and reactance at point "a" only. Supposing the linear output is positive on the left side of the line, and selecting W,, = +1 to minimize on-line calculation during faults, the three weights are:

w,, = 0 w,, = 1

X A ( 4 w,, = -- RA(')

A similar method is used to obtain weights for L3, L5, L7, L9, L11 and, L13. It is only slightly more complex to determine the weights for L2, LA, L6, L8, L10 and, L12. Taking L2 as an example and substituting the reactance and resistance of point "a" and "b" in Fig.5 into the equation

L, = W2, XA + W,R, + W, = 0

w2, ' A ( ' ) + w, RA ('> 't wzO =

(11)

gives

(12) w2,XA(b) + w n R A ( b ) + wzO = {

and, supposing that the linear output is required to be positive on the trip-region side and selecting W,, = 1, the weights are given by,

W2] = -1

WZ = (b) - xA (')

(13)

As stated previously the on-line calculation in this scheme consists of two parts, prior to a fault and during a fault. The

calculation of the weights of all the straight lines is done in the former part. The weights for the example study case of Fig.4 to Fig.6 are listed in Table I. The computation took only 9.7 milliseconds of 80286-16MHz processor time.

The network parameters, equivalent source potentials and impedances of the two ends, could be developed at the substation using a locally available computer system and used by the digital distance relay of each transmission line. Changes of the external equivalents are monitored and when they are beyond a certain limit the ideal trip characteristic is renewed and the weights recalculated.

Table I The weights for the study case

i

1 2 3 4 5 6 7 8 9 10 11 12 13

Wi,

1 -1 1

-1 1

-1 1 1 1 1 1 1 1

wi2

-14.300 -0.16692 -0.45456 -0.06348 -0.2649 8 -0.0001 1 -0.2 1320 -0.29751 -0.17510 -0.694 17 0.00523

0.03262 -0.03147

Wio

0 35.282 0

29.410 0

23.735 0

9.3812 0

39.783 0

2.0874 0

During fault, after fault detection and fault classification, a digital distance relay calculates the apparent reactance and resistance. From this the linear outputs L1 through L13 can be obtained. The positive or negative value of these outputs and certain "OR" and "AND" logic relations between them are used to reach a final trip decision as shown in Fig.7.

tripping decision

Figure 7 Trip decision logic from the linear outputs (the linear outputs are true when they are positive)

It is obvious that the computational task for the processor is small. In this study case the post-fault on-line calculations are no more than 13 multiplications, 19 additions, and 12 logic operations. This involves relatively little real-time digital processing and is well within the capability of presently

485

then the fault type is determined. Apparent reactance and resistance are calculated in the measurement unit. Generally, successive estimates of the impedance are compared with the boundary conditions at every sampling interval. To improve the performance of currently available digital distance relays a practical scheme shown in Fig.8 was designed.

available 16-bit microprocessors. Real-time testing on a 80286- 16MHz microprocessor shows the maximum processing time for this example was 0.395 milliseconds.

3. Tovoloeical Considerations

In practical application since the ideal trip region is simplified by linear approximation to the four curved-boundary lines, I, II, ID, N, the following points are pertinent. Fault Fault

(1) Line I: Since measured reactance and resistance of solid faults follow this trail, it is important that for clearing solid

- detec-- classi- - tion fication ment

faults line 1 itself is unambiguously included in the trip region. A positive threshold on the corresponding linear output will make this possible.

Figure 8 Block diagram of a digital distance relay with the adaptive setting scheme

(2) Line II: Measured reactance and resistance of faults located at the end-of-relay-reach point with different fault resistance will follow this line. Therefore, increasing the number of straight lines used to replace this curve will be beneficial for reach accuracy of relaying.

(3) Line III: Measured reactance and resistance of faults that include same fault resistance at different location will follow this line. In general, it is not necessary for a distance relay to trace this curve with great accuracy and, therefore, a smaller number of line segments can be used so as to reduce the computation burden. However, if clearing high resistance faults within the protected region is a desired feature of the relay, a higher maximum fault resistance will be selected in the determination of the trip boundary, so long as false tripping caused by system conditions such as maximum load current are avoided.

(4) Line N: Faults at the switch gear location with different fault resistances will follow this line. Directivity of the relay should be taken into account.

4. Consideration for effects of the VT and CT Tolerances

In general two factors limit reach setting in distance protection-- i) the combined far end infeed fault resistance problem, ii) VT and CT accuracy. The scheme as described so far has emphasised how the former problem is overcome, but the second may appear to remain. However, since the topic of crucial importance here is far end, high resistance faults, fault currents will be smaller and the usual problem of current transducer saturation errors will be less severe. Furthermore, by removing the first problem the way is now open to fully exploit the advantages of accurate optical current and voltage measurement devices. A zone 1 setting of 95% will leave a small margin for residual measurement and calculation errors,

In the block diagram, an adaptive setting (AS) unit works in collaboration with a default setting @ S ) unit. The default setting OS) unit with a pre-set conventional quadrilateral characteristic remains valid during a fault until its recovery. The adaptive setting (AS) will only be unblocked for a certain period during a fault. One reason for this is that the accuracy of the measured impedance is adversely affected by transient components in the first cycle of a fault and using the adaptive setting with an almost whole line boundary may cause maloperation. Another reason is that false tripping may also be caused by the removal of the remoteend infeed. Time counting from the instant of fault inception is initiated by the fault detection unit. Both high speed and good selectivity are obtained by using these two setting units together.

Different fault types have different ideal trip regions. Different weight patterns for different faults are calculated prior a fault. The fault classification unit is used to select the weight pattern appropriate to the fault.

Computer simulation of the scheme was conducted on a two- terminal system simulation model and it was clear from Fig.2 and Fig.3 that the improvement of selectivity and sensitivity was significant. For speed testing, transient fault data was generated by a transient-waveform simulation program which was developed using reference [4]. A one-cycle window relaying algorithm was used and the AS unit was unblocked 20ms after the faults occurred. The zone-1 reach was set to 95% of a 128km line. Different arc resistance faults (0--2003) under different system conditions were simulated and it was found that a trip signal could be released within about 20.4ms (20+0.395ms). It is apparent that the proposed adaptive scheme does make a digital distance relay operate with improved sensitivity, selectivity and speed.

This new adaptive setting concept is also suitable for multi- terminal lines although only a two-terminal line model has been IMPLEMENTATION A discussed in this paper.

Extra-high-speed digital distance relays have been implemented with different algorithms and some can operate within one cycle or even a few milliseconds of fault occurrence. A typical digital distance relay consists of a fault detection unit, a fault classification unit, a fault measurement unit and, a trip region comparison unit. A disturbance or a fault is first detected and

CONCLUSIONS

An adaptive relay setting =heme for stand-alone digital distance protection has been proposed in this paper. From both hardware and software viewpoints it is intended for incorporation with

486

today's microprocessor based commercial protective relay technology. Network conditions are monitored and setting patterns are renewed automatically according to network configuration changes through a pre-fault calculation routine. When a fault occurs the relay will operate with this ideal trip characteristic. This scheme involves relatively little real-time digital processing. Sensitivity, selectivity and speed have all been improved by using this scheme in a digital distance relay.

ACKNOWLEDGMENTS

The authors would like to thank the Hong Kong Polytechnic for a research grants and the Dept. of Electrical Power Engineering & Automation, Tianjin University for technical support.

APPENDIX

Referring to Fig.1 the prefault load current in phase-A can be expressed as

(14) E,-E, - (1 -he-j6)E,,

I , = - - '1, + ' I N '1, + ' I N

and the prefault voltage at F is

U, = E,, - I,Z1, (15)

During a single-line to ground fault in phase-A, the positive-, negative- and zero-sequence currents through the fault resistance R, are

Thus the sequence currents from M to F are expressed as

I = I = C I =- Cl U,, I, I IF Z , +3RF

CO UAFD I , = C I =- OF Z , + 3 R ,

L =

where CO and C, are the positive- and zero-sequence distribution factors defined by

Therefore the phase-A current IAM may be synthesized from

I,, = I,, + I,,, = I,, + (Il, + 12M + I,,) (21)

and the phase-A voltage at relaying point M may be obtained from

U, = ( I , + IT + I,) RF + ( I , + l M ) Z , m + IMZW + I,Z, (22)

Consequently the apparent impedance of phase-A measured at M is

where I<oL is zero-sequence compensating factor,

ZOL - ZIL KOL = - 3 Z1L

where Z,, and Z,,, are the positive- and zero- sequence impedance of the protected line.

Using these relations and after algebraic manipulation, it can be shown that,

3% (25)

Z,=Z,+ (ZI: +3R,)(1 -he-'*)

ZlN+hZ,ei6 + 2C1+C,(1+3K,)

REFERENCES

[l] G. D. Rockefeller, C. L. Wagner, J. R. Linders, "Adaptive Transmission Relaying Concepts for Improved Performance", IEEE Transactions on Power Delivery, Vo1.3, No.4, pp.1446-1458, October 1988.

[2] A. K. Jampala, S. S . Venkata, M. J. Damborg, "Adaptive Transmission Protection: Concepts and Computational Issues", IEEE Transactions on Power Delivery, vo1.4, N0.1, pp.177-185, January 1989.

[3] Zhang Zhizhe, Chen Deshu, "An Adaptive Approach in Digital Distance Protection", IEEE Transactions on Power Delivery, vo1.6, No. 1, pp.135-142, January 1991.

[4] A. T. Johns, R. K. Aggarwal, "Digital Simulation of Faulted E.H.V. Transmission Lines with Particular Reference to Very-High-speed Protection", IEE Proc. Pt.C. Vo1.123, No.4, pp.353-359, April 1976.

BIOGRAPHY

Xia YanOuan received the B.Sc. degree, and the M.Sc.(Eng.) degree in Electrical Engineering from the Tianjin University, Tianjin, China, in 1986, and 1989 respectively. In 1989 he joined the faculty of the Tianjin University. Since 1990 he has been engaged on a collaborative program in digital protection research at the Hong Kong Polytechnic and is working towards his Ph.D. degree. His research interests are in power system protection & control and computer applications in power system.

K. K. Li (M '76 - SM '91) received the M.S. degree from University of Manchester Institute of Science and Technology, UK in 1971. From 1972 to 1979, he worked as a Protection Engineer in the Hong Kong Electric Co. Ltd. and the China Light & Power Co. Ltd., Hong Kong. In 1979 he joined the Hong Kong Polytechnic and is currently a Senior Lecturer. His research interests are mainly in power system protection.

A. Kumar David (M '89) obtained his B.Sc.(Eng.) from the University of Ceylon in 1963, and his Ph.D and D.1.C from Imperial College, University of London in 1969. He has worked in Sri Lanka, Zimbabwe, USA and Hong Kong and his research interests are in a variety of power system topics.

487

discussor’s view that zone 1 distance relays cannot be expected to approach the behavior of a pilot relaying system, and compli- cation of the zone 1 measurement technique does not seem justified. Manuscript received February 10, 1993.

Discuss ion

Mike MEISINCER (Associated Relay Technologies, Roswell, GA):

The authors indicate the adaptive satting (AS) unit will correctly adjust its characteristic in accordance with the influences of pre-fault load flow on a resistive fault based on system data derived from a relatively slow speed SCADA link. The prevention of overreaching and underreaching for this condition is recognized as being highly desirable.

Where the pre-fault load is from the near bus to the far bus (producing an apparent fault impedance which is closer to the near bus), I would appreciate the authors elaborating on how the AS unit is adjusted to prevent underreaching during a single ended high speed (30 cycle) tripping and reclosing sequence initiated from the near bus for a high resistance fault which falls outside the reach of the default setting (DS) unit. Specifically, will fault detection and clearance now be dependent upon the speed with which the relay is updated to the final system configuration resulting from the trip and reclose sequence? Additionally, what is the impact of changing system data, which reflects the tripping and reclosing sequence, on the AS unit?

The authors also recognize the difficulty of calculating positive and zero sequence impedance of double circuit lines on the same tower. Would the authors please expand on how the AS unit is adjusted for this condition? Also, would the authors please address how partial mutual coupling and mutual coupling resulting from a parallel line grounded at both ends is accommodated?

Manuscript received December 4, 1992.

Walter A. Elmore, ABB Power T & D CO Inc, Coral Springs, Florida: The authors are to be congratulated on the clarity of exposition in this paper. It addresses the problem of identifying fault location on a very fundamental basis, and yet proposes an interesting variation of the present relaying art.

The method described appears to place dependence on a knowledge of the source impedances at each end of the line which are “updated from local circuit impedance, breaker status information, and on-line measurements of system load.” More explanation of how this information is to be acquired and used would be of interest. Also, in a complex network, with parallel circuits between busses, the fault location changes the equiva- lent source impedances (as well as Zlsm/Zosm ratio, for example), and we wonder what effect this will have on the estimate of fault location, using the method described.

All of the authors’ efforts have been directed toward the influence of varying system parameters on apparent impedance as viewed by a zone 1 relay. While this is a very useful theoreti- cal exercise, is it not more realistic to place predominant respon- sibility for high speed detection of fault location on pilot relays which utilize a communications channel to circumvent the defi- ciencies of zone 1 relays and to do so with far less demanding setting criteria? Further, a pilot scheme (of any type) will pro- vide high speed, sensitive, selective nonsequential response to all faults at all locations on the protected line. This is a require- ment that no zone 1 relay, irrespective of refinements, can possibly accomplish.

This is not intended to detract in any way from the excellent contribution made by the authors, but it has long been this

J. Pinto de 8hflechnical University of Lisbon, Portugal): There is no doubt that the investigation of methods to adapt the operating characteristic of distance relays to the System conditions is to encour- age. This is because augmenting the reach of zone-1 without loss of security can avoid the capital and maintenance cost of tele-protection schemes. In addition, such schemes do not seem to require heavy real-time processing capabilities. On the other hand, interfacing stan- dards to link Intelligent Electronic Relays to RTUs are being devel- oped. so that the general approach of this paper may be quite feasible in the medium term, from a technology point of view. However, the “Adaptive Setting Fundamentals“, as introduced by the authors from page 2 on, start with a critical error: the scheme of Fig- ure 1 can only model reality in very particular conditions. The correct and general model for scheme (positive, negative and zero) is as in Figure D1, (see Dl], pages 4446-4447):

Figure D1 - The correct fault model where Zl and Y1 are respectively the line longitudinal impedance and transverse admittance, d is the portion of that line from the relay to the fault locations, and Z M ~ , ZNS and Z M N ~ can be remeved from the related elements of the node impedance mamx ZMM, ZNN and ZMN as follows:

@3) 1 1 1

A very important aspect to consider, particularly for long lines, is the capacitance of the line lumped at the relay location, which is itself as in the model a function of the fault distance. Consideration of this better model can lead to substantial real time processing requirements, and I question the authors whether it would not be better to leave the task of computing the whole operating characteristic to some Computer Aid linked to the SCADA system. That Aid would always have to exist to compute the actual node impedances employed by the relay, so why not the all data?

However, a more fundamental point is to be commented regarding this paper. An adaptive operating characteristic can improve the reach of distance relays for high resistance faults, thus supporting single phase reclosing (something hard to implement if zero overcurrent re- lays are employed, as usual for high resistive faults). This adaptivity, together with the improved speed of distance relays, can minimize the

488

very common evolution of such faults to multi-phase low-resistance faults. However, it can not overcome a set of other sources of inaccuracy that the authors seem to neglect, namely: 1 - The zero impedance of the line, which is strongly dependent on

the ground resistivity, which in turn depends on the ground hu- midity and as such on the wether, a random variable.

2 - CT and VT measurement errors, which do not arise only with sat- uration (although with it they may become out of control); further- more, optical transducers also exhibit dynamic errors.

3 - The error of the relay itself, which as a rule is only guaranteed for a 5% limit and even that without accounting for electromagnetic transients.

4 - The charging current of the line, which also depends on the bus voltages.

References [Dl-D2] are good surveys of the relative importance of a few sources of error as these for the accuracy of distance relays. If such sources of error are considered, may be the operating character- istic of Figure 4 will have to be surrounded by a simplified larger characteristic, whereas the 95% setting will have to be reduced to the usual 80 or 90%. This may well be the rediscovery of the quadnlat- eral operating characteristic usually employed in some counmes since a long time ago (countries where static relays were widely adopted since the 1970's). The higher reactance segment of this polygon, however, can surely be made more adapted to the phase angle between the current conm- butions from the two ends of the line. For high resistance faults the superposed load current is a significant portion of the line current, and because it usually is quite in phase quadrature with the fault cur- rent it is a major responsible for that phase difference. Therefore the approach of reference [3] can be much more realistic than the one presented in this paper, not to mention its no need for real time up- dated network information, making immediate its feasilibity. But of course the availability of additional network data from a Control Center can still marginally improve the accuracy of that adaptive rota- tion of the upper limit of a polygonal impedance characteristic.

References: [Dl] - E. R. Sexton, D. Crevier, "A Linearization Method for deter- mining the effect of Loads, Shunts and System Uncertainties on Line Protection with Distance Relays", IEEE Trans. on PAS-100, N2. 11, November 1981, pp. 4439-4447. @2] - T. Rodolakk, D. Crevier, "Effect of Loads, Shunts and Sys- tem Uncertainties on Short Circuit Relays Settings", EEE Trans. on PAS-100, N?. 12, December 1981, pp. 4701-4709.

Manuscript received February 25, 1993.

B.Jeyasurya, M.A.Rahman (Memorial University of Newfoundland, St.John's, NF, Canada) : The authors are to be commended for presenting an adaptive setting concept for digital distance relays. The authors' comments on the following points would be welcome.

1. The computation of the setting boundaries need measurements from both ends of the transmission line. To determine the angle ' 6 , (Equation 5 ) , the sampling of voltages and currents at both the terminals should be synchronized. Would the authors outline how this can be achieved?

2. The proposed adaptive setting unit introduces an intentional delay of one cycle. Will this not degrade the performance of many short data window algorithms [A]?

3. Many algorithms are available to correct

the error in distance relays due to the finite fault impedance. If data from both ends or even from one end are available the actual transmission line impedance up to the location of the fault can be determined [B,C]. The relay can respond to the changing network conditions even with fixed settings.

Reference

[A] A.G.Phadke, J.S.Thorp, "Computer Relaying for Power Systemsg', Research Studies Press Ltd., England, 1988.

[B] L.Eriksson, M.M.Saha, G.D.Rockefeller, "An Accurate Fault Locator With Compensation for Apparent Reactance in the Fault Resistance Resulting from Remote-End Infeed", IEEE Transactions on Power Apparatus and Systems, Vol. PAS- 104, No.2, February 1985, pp. 426-436.

[C] B.Jeyasurya, M.A.Rahman, *ISimulation of Transmission Line Fault Locators in a Personal Computer", IEEE Transactions on Industrial Applications, Vol 27, No.2, MarchIApril 1991, pp. 299-302.

Manuscript received February 25, 1993.

DAVID G. HART and DAMlR NOVOSEL, ABB-TTI, Raleigh, NC: Adaptive relaying is certainly one of the most important topics in power system protection today. In this paper, the authors have attempted to employ adaptive protection for transmission line protection. We offer the following comments.

(i). For a transmission line, the sending and receiving end equivalent networks are certainly governed by the system configuration and the line load. Varying the system configuration or line load will significantly affect these models. The terms E,, and E,,,, (preceding equation 2) are related by the amplitude ratio and the power angle. The authors do not mention telemetering of voltage or current data from the remote end or obtaining this data from SCADA, only the use of the attenuation factor (h e*). How is this parameter computed and updated? Do the authors assume constant emfs behind each equivalent system reactance that are proportionally related by (h e*)? In light of Figure 3, this does not seem to be the case. I f computed on-line, how often must this value be determined, and what inputs are required? If voltage and current data is required, why not use this data directly?

(ii). Equations 3, 4, and 5 are related to single-line-to-ground faults. For a given complex (h e*), 2, and 2,. the apparent impedance is shown in Figures 4 and 5 for a varying R,. Are the results shown in these figures for single-line-to-ground faults or are all fault types included? If this is for single-line-to-ground faults only, how are other types of faults handled by the algorithm? Is a 'trip region' required for each fault type?

(iii). The convexlconcave 'trip region' is subdivided in Figure 5. As illustrated in Figure 5 and Figure 6, the authors use the apparent impedance to determine the trip signal. Figure 6 shows how a 'positive or negative' output is given, depending on whether the apparent impedance is within a subregion or not. Figure 7 shows that the trip signal is given if the fault is determined to be exclusively in one subregion. Would the authors elaborate on how the algorithm determines the subdivisions with fast system changes. Would this be done on-

line or off-line for each system change? For instance, how would the algorithm respond to closing on a fault on adjacent line (varying configuration vs real-time update)? Furthermore, removal of remote-end infeed typically enhances relay operation. An ordinary reactance algorithm will compensate for the fault resistance and should easily be capable of detecting the fault. Does this condition adversely aff act the algorithm because the scheme cannot properly update the 'trip region'?

(iv). It appears that the scheme may not be secure, particularly around boundaries II and 111. Have the authors tested the scheme with external faults with power flow in both directions, and under various load conditions? How will parameter uncertainty (particularly zero sequence) and parallel lines influence the accuracy of the scheme?

(v). In section 4, the authors state the algorithm overcomes VT (CVT?) problems. Would the authors elaborate on this point?

In summary, the authors paper certainly is interesting, but elaboration is necessary. The practical implementation and justification for the algorithm does not seem to be supported. Any comments the authors wish to make would be appreciated.

Manuscript received March 8, 1993.

Y.Q. Xia, K.K. Li and A.K. David: We thank the discussers for their interest, useful comments and questions. At the outset we must say that the paper did not directly concern itself with the details of how the external equivalents were to be established and confined itself to the issue of exploiting an external equivalents model for enhancing the performance of distance relays by adaptive methods. However, it is clear from the questions raised, that some further discussion of equivalents modelling is required.

Mr Elmore and Professor Pinto de Sa have drawn attention to the inadequacy of the model in fig. 1 of the paper, which treats the external equivalents at the two terminals as distinct, in cases where there are other parallel lines and substantial alternative interconnections between the terminals. We can assure Professor de Sa that this was not a "critical error" but a simplification that we were very aware of. Where significant interconnection needs to be modelled, using the nodal impedance methods of network theory, we propose the modified external system model of fig.9 (Professor de Sa's fig.D1 without the shunt capacitance). The crucial point, however, is that the use of this slightly more complex network model has insignificant effect on the adaptive relaying method described in the paper. Certainly, equations (3) to (5) of the paper and the distribution factors in the Appendix will be altered, the pre-fault on-line processing will be a little more complex, but neither the complexity nor the time for the critical during-fault computations are affected. We have established this by numerous computer simulation exercises of the fig.9 system for different fault locations and resistances for various initial conditions. If line shunt arms are included the prefault computation time is further increased but not the during fault computations. Compared to the time required to update the system SCADA data base a time increase of milliseconds to reset the trip boundaries is irrelevant. We envisage that the system model is developed on a system level host computer (Professor

489

Pinto de Sa's Computer Aid) since it is common to all portion of the power system and useful for other adaptive relays as well (51. However, the specialized task of establishing the trip boundaries localized to each adaptive distance relay is more suited to microprocessor based stand-alone relaying clusters.

I I 1

Figure 9 Modified external system model

We will now turn to several other specific questions raised by discussers. Mr Elmore makes some pointed comments regarding the advantages of pilot relaying and obviously, if the additional funds were committed, these advantages could be enjoyed. However, it has to be recognised that in numerous circumstances pilot relaying will not be employed and in that case the enhancement of the performance of distance relays is a worthwhile objective.

In response to the other matters raised by Professor de Sa we begin with the question of additional "substantial real time processing" required if effects such as line capacitance are to be included in the model. Not so, in other studies, we have considered the incorporation of factors such as line capacitance and mutual coupling with parallel lines on the same tower. The effect, always, is to increase the pre-fault on-line computation requirement by a few milliseconds with little change in the sub- millisecond during-fault decision making time. Essentially, these factors introduce a little more complexity in setting up the adaptive region (boundaries) but do not materially affect the decision making about whether a measured impedance point lies inside or outside the trip region when a fault occurs.

Professor de Sa is also of the view that we have neglected four sources of inaccuracy and that this makes a striving for 90 or 95 % zone 1 reach setting impractical. First we wish to comment that we do not understand why "the error of the relay itself" should be as much as 5% with digital protection. Second we mention that the incorporation of line capacitance has been undertaken as discussed in the previous paragraph. Third regarding the variation of zero sequence impedance magnitude with ground, and therefore random weather conditions, it has to be observed that in so far as this mainly affects the real part of the line zero sequence impedance the uncertainty already considered in the modelling of a wide range of fault resistance values effectively addresses this concern.

490

VT and CT errors raise both difficulties and possibilities. If their combined effect cannot be held to less than 2 or 3% in unsaturated conditions we agree that relay performance will be affected. Obviously, saturation effects are less pronounced with high resistance faults which are the principal concern of this paper. An important consideration for the future, however, is that in conventional distance relaying there is little motivation for employing more accurate and expensive CTs and VTs if in any case one is faced with significant inherent errors due to remote end infeed. If however the latter limitation is overcome it becomes worthwhile to invest in more accurate transducers.

Drs Jayasuriya and Rahman inquire how the angle 6 is to be obtained. The ratio h and the angle 6 are derived pre-fault, on- line, from time to time (whenever the relay adaptive settings are to be updated), by using information available in the system on- line SCADA database. If the voltage magnitudes and line flows in all parts of the system are available the phase angles between pairs of voltages, or the phase angles behind system equivalent impedances, can always be computed. We did not anticipate time synchronised system wide angle measurements but it might be relevant to mention at this point that the availability of synchronised sampling techniques accurate to micro-seconds made possible with satellites [6] will make it possible for future SCADA systems to implement system wide phase angle measurements against a universally available clock. To respond to the question "why not use the voltage and current data directly" we would like to say that minimising the on-line prefault and during-fault computations is a major objective of this scheme.

Drs Jayasuriya and Rahman also inquire what advantage the technique proposed in this paper has in comparison with methods which process on line data to make accurate assessments of fault location. The advantage is that the proposed method does most of the processing pre-fault when it adaptively establishes the trip regions shown in fig.4. During fault only minimal computation needs to be done, simply to ascertain whether the measured impedance lies within or outside the most recently updated boundaries.

The extended zone 1 coverage demands accuracy hence a one- cycle window length algorithm is used to eliminate uncertainty due to transients. It is not an intentional time delay. In order not to degrade the merits of short window algorithms the default setting unit (DS) is of the conventional type and has zone 1 reach and fault resistance accommodation limited to conventional values.

Next we will respond to the matters raised by Messrs Hart and Novosel. Yes, indeed we envisage that telemetered or SCADA system data will form the information base from which the equivalent circuits of the external system are developed. The advantages of the adaptive approach compared to using the data directly in on-line during-fault processing has been discussed in the previous paragraph. We do assume constant voltage magnitudes and angles, behind constant reactances, between "adaptations". The trip regions and their boundaries are adapted (updated) on-line from time to time. There need be no fixed

time interval at which the boundaries are updated and will be done only if there is a sufficient difference between the currently "active" version and an update proposal. Line switching in other parts of the system in the vicinity of the protected line or significant changes in the system loading conditions may modify the external equivalents sufficiently to make an adaptive update of the currently active trip boundaries necessary. An update need be implemented only when it is necessary. Therefore the update implementation interval may only be once in several minutes or even hours although update proposals may be examined at much more frequent intervals.

In response to Hart and Novose' question (iii) we wish to say that it is not necessary to determine which subdivisions a fault is located in. The prefault computation time of 9.7ms is required for deriving the weighting parameters. The during-fault time of 0.395ms is the trip decision making time. Regarding question (iv), the scheme has been extensively tested by digital simulation for both external and internal faults with power flows in both directions for a variety of source voltage and impedance conditions at both terminals and performance has been found to be satisfactory. The topological trip decision logic scheme has proved to be satisfactory for power flow in either direction. For the case of heavy load from the remote to the local end combined with extreme source conditions it can be difficult to secure accuracy around the boundaries II and III if a very high fault resistance accommodation is also demanded. The tradoff between calculation burden and boundary accuracy should be considered taking into account the processing capacity of the microprocessor to be used.

Regarding different fault types we wish to point out that the apparent impedances shown in figs. 4 and 5 refer to single-line- to-ground faults only. This is the case when the combination of remote end infeed and high fault resistance is most serious. If the principle of adaptive relaying is to be extended to other classes of faults, using a system externals model as described in the paper, it is quite feasible to do so but a separate trip region will be required for each type of fault and the appropriate one would be selected by a conventional fault classification method.

Finally we will discuss the matter raised by Mr Meisinger and in Hart and Novosel's question (iii). In its most general terms this question may be stated as follows: "If the relay is called upon to make a decision soon after some event occurs in the system but before the on-line adaptive resetting feature has had an opportunity to activate a new set of trip boundaries, what will happen?" Two possible variants are illustrated in fig. 10 where the solid line represents the currently active boundary and (a) or (b) what the reset boundary should be just after the event. In case (a) the relay may over-reach and this is the reason for blocking the AS unit after about 5 cycles when switching elsewhere may alter the system configuration. In case (b) the relay under-reaches and some of the advantage to be gained from the adaptive scheme is not realised. A third variant incorporating features of both (a) and (b) where either over- or under-reaches depending on fault location and resistance is also possible. We give below some circumstances in which (a) or (b) may occur.

49 1

The important practical consideration is, in which circumstances is there a reasonable likelihood that the relay may be called upon to make a decision before an adaptive update is implemented? It is worth recalling that the time required to carry out the on- line pre-fault updating of the boundaries is in subsecond, hence, theoretically, it is possible to adapt the trip region several times each second. Hence fast changing system conditions or switching actions elsewhere in the system can be captured at an adequate speed. The true limitation is that there in no point in attempting to make adjustments faster than the rate at which the SCADA data base is updated and this places a ceiling on the rate at which the external equivalents can be recomputed. However, in the case of "millisecond problems", such as auto-reclosing on to a fault while the far end status remains changed, or the Occurrence of a fault in or outside the protected line during or immediately after fault clearing in the vicinity, it is unlikely that the adaptive settings can be changed before the relay is called upon to act. In any case as mentioned before the AS is blocked after a few cycles and hence the danger of overreaching (case (a) situations) is minimized.

R-X Plane I

40 eo 120 160 zbo I

Figure 10 Variation of ideal trip boundaries

Case (a):

remote infeed open; local equivalent source becomes stronger or the remote becomes weaker; power flow reverses from N -. M to M -. N; reclose on fault with remote infeed still open; ............

case (b):

local equivalent source becomes weaker or the remote becomes stronger; power flow reverses from M -. N to N -. M; ............

REFERENCES

[5] IEEE Working Group K8 of Power System Relaying Report, "Feasibility of Adaptive Protection and Control", IEEE PES Summer Meeting 1992.

[a] IEEE Working Group H7 of Power System Relaying Report, "Synchronized Sampling and Phasor Measurements for Relaying and Control", IEEE PES Winter Meeting 1993,93 WM 037-2-PWRD.

Manuscript received June 21, 1993.