Adaptive Current Differential ProtectionSchemes.pdf

1832 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 4, OCTOBER 2009

Adaptive Current Differential Protection Schemesfor Transmission-Line Protection

Sanjay Dambhare, S. A. Soman, Member, IEEE, and M. C. Chandorkar, Member, IEEE

AbstractThroughout the history of power system protection,researchers have strived to increase sensitivity and speed of ap-paratus protection systems without compromising security. Withthe significant technological advances in wide-area measurementsystems, for transmission system protection, current differentialprotection scheme outscores alternatives like overcurrent and dis-tance protection schemes. Therefore, in this paper, we address thischallenge by proposing a methodology for adaptive control of therestraining region in a current differential plane. First an erroranalysis of conventional phasor approach for current differentialprotection is provided using the concept of dynamic phasor. Sub-sequently, we extend the methodology for protection of series com-pensated transmission lines. Finally, we also evaluate the speedversus accuracy conflict using phasorlets. Electromagnetic Tran-sient Program simulations are used to substantiate the claims. Theresults demonstrate the utility of the proposed approach.

Index TermsAdaptive protection, current differential protec-tion, dynamic phasor, global positioning system (GPS), mutuallycoupled lines, phasorlets, series-compensated lines, tapped lines.

I. INTRODUCTION

I T is a well recognized fact that differential protectionschemes provide sensitive protection with crisp demar-cation of the protection zones. In principle, the differentialprotection is also immune to tripping on power swings. Suchschemes when used for transmission systems using pilot wiresare called pilot relaying schemes [1]. In 1983, Sun and Ray[2] published a seminal paper describing current differentialrelay system using fiber optics communication. An effectivetransmission rate of 55 samples per cycle at 60-Hz frequencywas achieved in [2]. Since, differential comparison of thelocal and remote end currents must correspond to the sametime instant, a delay equalizer is used with the local sequencecurrent component signal to compensate the delay in receivingthe remote end currents.

The inaccuracies in such a current differential protectionscheme arise, primarily, due to the following reasons:

effect of the distributed shunt capacitance current of theline is neglected;

modelling inaccuracies with series-compensated transmis-sion line;

Manuscript received May 30, 2008; revised December 13, 2008. Current ver-sion published September 23, 2009. This work was supported by PowerAnserLabs, IIT Bombay. Paper no. TPWRD-00395-2008.

The authors are with the Department of Electrical Engineering, Indian In-stitute of Technology Bombay, Mumbai 400076, India (e-mail: [email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPWRD.2009.2028801

approximate delay equalization between local and remoteend current;1

current transformer (CT) inaccuracies, in particular errorsdue to saturation of the core in the presence of decaying dcoffset current [3].

Conventional current differential schemes employing GPS-synchronized current measurements are discussed in [4][6]. Ifultra high transmission system voltages are used (e.g., 765 kVand above), then line charging current component is significant.It causes a large variation in phase angle of the line current fromone end to another. In traditional pilot wire schemes, relayingsensitivity will have to be compromised to prevent the mal-oper-ation. Reference [7] proposes a current differential relay whichuses distributed line model to consider line charging current.An adaptive GPS-synchronized protection scheme using Clarketransformation is proposed in [8]. The multiagent-based widearea current differential protection system is proposed in [9].References [10], [11] propose the use of phasorlets for fast com-putation of phasors in distance and differential relaying. Analyt-ical treatment of phasorlets is presented in [12].

If a transmission line has a series capacitor, then depend-ability of the conventional current differential relay may be com-promised due to current inversion. Current inversion dependson degree of compensation, fault parameters and metal oxidevaristor (MOV) conduction. MOVs have nonlinear V-I charac-teristics and they are connected in parallel with series capacitorsto protect them from overvoltage. The performance of transmis-sion system protection scheme in presence of series capacitor isdiscussed in [13]. In the case of distance relaying, effect of seriescompensation is even more serious due to presence of current orvoltage inversion [14][16]. Even segregated phase comparisonscheme may fail to operate on current inversion [17].

The paper on digital communication for relay protection [18]authored by working group H9 of IEEE Power System Relayingcommittee is an excellent reference to understand the impli-cations and consequences of digital communication technolo-gies on relaying. Modern high speed communication networks,typically use Synchronized Optical Network (SONET) or Syn-chronized Digital Hierarchies (SDH) standard for communi-cation with transmission rates of the order of 274.2 Mbps or155.5 Mb/s, respectively. They permit network protection,that is, during failure of a communication link, communica-tion services are restored by reconfiguring flow of informationin alternate paths. A typical example is self healing ring archi-tecture used with SONET [19]. In such networks, synchroniza-tion by delay equalizers become difficult due to channel asym-metry. Due to channel asymmetry, communication delays for

1For example, at 50 Hz an uncompensated delay of 1 ms in communi-cation will translate into an error of approximately 13 degrees in the phasecomputation.

0885-8977/$26.00 2009 IEEE

DAMBHARE et al.: ADAPTIVE CURRENT DIFFERENTIAL PROTECTION SCHEMES 1833

transmit and receive paths are not identical. This may lead todifferential currents arising out of inaccurate delay equalization,especially if, identical time for transmit and receive paths areconsidered. However, if current samples are time stamped bya global positioning system (GPS), then for calculation of dif-ferential current, samples corresponding to the same time in-stant can be compared, thereby providing immunity to channeldelays, asymmetry, etc. [5], [20]. Differential current may becalculated either using instantaneous sample values, or by ex-tracting phasors. Further, dynamic estimate of the channel delaycan be easily maintained by subtracting the GPS time stamp atthe transmit end from the receiving end time stamp. This per-mits back up operation even during GPS failure modes.

The primary objective of this paper is to propose a method-ology to improve sensitivity and speed of the current differen-tial protection scheme for transmission-line protection withoutcompromising its security. To meet this objective, we first de-velop a dynamic phasor model of transmission line. The modelhelp us to analyze errors associated with steady-state phasormodel of transmission line. Two parameters and are de-fine to quantify errors arising out of neglecting dynamic phasors.Subsequently, we propose an adaptive procedure to set the re-strain region in the current differential plane. We show that theproposed methodology significantly improves sensitivity andspeed of the current differential protection scheme without sac-rificing the security.

This paper is organized as follows: a current differentialprotection framework is introduced in Section II. In Section III,a dynamic phasor model of transmission line is developed. It isused for understanding modelling errors in current differentialprotection scheme. Consequently, in Section IV, the idea ofadaptive restrain region is developed. Section V explains theimplementation in phase co-ordinates. Section VII extends itto series-compensated and multiterminal lines. In Section VIII,we present simulation case studies in EMTP-ATP packageon a 4-generator, 10-bus system with the capacitance coupledvoltage transformer (CCVT) model. Section IX concludes thepaper.

II. FUNDAMENTALS

Let us consider the positive sequence representation of an un-compensated transmission line. As shown in Fig. 1, the line canbe represented by an equivalent -model. Equivalent circuitmodels the effect of distributed line parameters at the line ter-minals at the fundamental frequency.

Let the positive sequence component of line current for refer-ence phase measured at bus be given by . Then, current

, in the series branch of the -equivalent line model at nodecan be computed as follows:

(1)

where is the current in shunt path at bus andis the positive sequence voltage of reference phase . Simi-

larly, current in series branch at bus can also be computed.If there is no internal fault on the line, then

(2)

Fig. 1. GPS-synchronized current differential protection scheme with equiva-lent -model of line.

Hence, can be used as discriminant function to detect afault on transmission line. This approach has been suggested byPhadke and Thorp [21, p. 257].

With a conventional relay-setting approach, operating currentand restraining current for the current differential scheme

can be expressed as follows:

(3)and

(4)The percentage differential relay pick up and operate when

(5)(6)

where is a pick-up current and is the restraint coefficient. However, it has been shown in [22] that nu-

merical differential relay can be set more accurately in a currentdifferential plane. Using the phase and magnitude informationof series branch current, we calculate

ratio (7)

(8)In absence of an internal fault, we have

andAs shown in Fig. 2, this can be visualized in the current differ-ential plane by point at (180 , 1). Ideally, every point otherthan indicates an internal fault. However, even in the absenceof an internal fault, in real life the operating point may deviatefrom the point (180 , 1) due to following reasons:

1) synchronization error;2) delay equalizer error;3) modelling restrictions (i.e., assumptions, approximations,

or inaccuracies of the algorithm);4) ratio and phase angle errors of CT. These errors may be-

come significant when a CT core saturates because of largecurrents during an external fault.

Since GPS provides time synchronization of the order of 1sec, the synchronization error can be practically eliminated.Also, if the same time stamped samples of two end are pro-cessed, delay equalizer error can be eliminated. Further, explicit


Fig. 2. Trip and the restrain region in current differential plane.

modelling of the shunt capacitance of the line reduces the mod-elling errors. Therefore, we can reduce the width of the restrainregion in the current differential plane. We use for phaseerror2 and % for magnitude error in current differentialplane. Hence,and (refer Fig. 2). The corresponding value of

in conventional relay setting approach is nearly 0.43.

III. ERROR ANALYSIS USING DYNAMIC PHASORS

In principle, one can question the validity of phasor computa-tion in relaying algorithms, irrespective of whether it is distanceprotection or differential protection, because if the relaying de-cision has to be arrived within one cycle of the fault inception,then, the actual voltage and current signals can differ appre-ciably from a sinusoid. This motivates us to develop a dynamicphasor model of transmission line.

Fundamental equations used to describe propagation of a dis-turbance on a lossless single-phase transmission line are [21]

(9)

(10)

where and are the inductance and capacitance of the lineper unit length and is distance measured from relay location.

If and are assumed periodic, then, (9) and (10) canbe transformed to the well-known phasor model of line

(11)

(12)

where and denote fundamental voltage and current pha-sors respectively. The -equivalent circuit is an exact two-portequivalent of (11) and (12). In that sense, methods based upon

-equivalent model are equivalent to method proposed in [7]which uses an explicit long line model.

The extension of phasors for the dynamic situation is dis-cussed in [23]. In this approach, signal can be representedas follows:

(13)

2Reference [22] has suggested margin for phase angle error.

where Re denotes the real part, ,and represent a dynamic phasor at the th harmonic fre-quency. Then, dynamic phasor at fundamental frequency can beevaluated as follows:

(14)

Equation (14) indicates that well known phasor computationalgorithms like full-cycle recursive DFT, half-cycle recursiveDFT etc., actually compute a dynamic phasor. However, infor-mation of rate of change of phasor is not utilized.

Rate of change of a dynamic phasor is given by the followingequation:

(15)

Prior to a fault or a disturbance, . Subsequentto a disturbance, voltage and current signals are not periodic.Hence, a dynamic phasor should be preferred for modelling.Using (15), an appropriate phasor representation of (9) and (10)is given by

(16)

(17)

where

By comparing (16) and (17) with (11) and (12), we can concludethat inaccuracies in differential protection based upon phasormodel arises due to neglecting the dynamic phasor contribu-tion terms and in (16) and (17). Pa-rameters and define gain associated with dynamic phasormodel. Further, and do not directly depend upon the pa-rameters of the line. Under the steady-state condition, and

are unity. Hence, andmeasures the inaccuracies associated with the steady-state mod-elling of the system. Prior to a fault, the terms and areunity. After the fault, and will deviate from unity. As thetransients die down, and returns back to unity.

Figs. 3 and 4 shows the behavior of and for a severethree phase bus fault.3 We observe that the transmission-linesteady-state phasor model is erroneous immediately after a fault.For a severe fault, model accuracy improves within two cycles.Fig. 5 shows variation of and for a less severe fault. Inthis case behavior of and shows that steady-state modelprovides a reasonably good approximation of system behavior.The behavior of error terms and suggest that thresholdparameter used to detect fault in differential protection shouldhave adaptive parameters. When model inaccuracy is high, thenrestraint should be high and vice versa.

3The system details are provided in Section VIII.


Fig. 3. Variation in and at end (refer Fig. 1) for phase . The externalLLL bus fault is at bus on 230-kV system (fault resistance is 0.1 and faultoccurs at 0.115 sec). Observe that both and are affected by a close in busfault.

Fig. 4. Variation in and at remote end (refer Fig. 1) for phase . Theexternal LLL bus fault is at bus on the 230-kV system (fault resistance is 0.1 and fault occurs at 0.115 s). Observe that remains close to unity for theremote bus and the behavior of is similar to at bus .

IV. ADAPTIVE CONTROL OF RESTRAIN REGIONA protection engineer strikes balance between dependability

and security of a relay by controlling the sensitivity. Depend-ability of a relay can be improved by increasing the sensitivity.Sensitivity of the differential relay can be improved by reducingthe area of the restrain region in the Fig. 2. Since, we have al-ready tightened the width of the restrain region, this implies thatwe should reduce height of the rectangle representing the re-strain region. However, it is equally important to keep it largeenough so that relay does not pick up on transients or externaldisturbance which includes a fault. Too sensitive relay settingincreases the possibility of relay maloperation and hence com-promises security.

We now propose an important enhancement to improve sensi-tivity of the current differential relay without compromising onits security. Basically, sensitivity implies an ability to detect lowcurrent or high impedance fault. The proposed enhancement isbased on the following observations:

Fig. 5. Variation in and at end (refer Fig. 1) for phase . The externalLLL bus fault is at bus on the 230-kV system (fault resistance is 100 andfault occurs at 0.115 s).

1) high impedance fault may not involve appreciable tran-sients (refer Fig. 5);

2) high impedance faults should not lead to gross errors dueto CT saturation and

3) large disturbances (e.g., load throw off and external faults)will cause large differential currents because 1) the phasormodel is not truly valid under such situations (as shownin Fig. 3) and 2) CT errors may increase due to partialsaturation; hence, large transients or disturbances demanda larger restrain region.

The aforementioned observations suggest that the height ofthe restrain region should be a function of the current magni-tudes of and . In particular, we propose the followingrestraining function:

(18)

where and are suitable constants. We assume thatis greater than 1. In case the ratio is less than

one, then the numerator and the denominator should be inter-changed. The relay trips when either: 1) the restraining functionis greater than zero or 2) when the angular separation criteriondescribed in the earlier section is violated.

1) Selection of Constants and : Under no-fault andsteady-state conditions, ratio is equal to one.Further, the term depends upon line currentand hence it can be very small ( and ). Thissuggests that constant should be at least equal to 1. Further,for sensitive protection, constant should be chosen close tounity. In particular, we have found and 0.0015to be a satisfactory choice. Small magnitude of is chosenbecause fault current range is approximately in kiloAmperes.At lower values of , possibility of relay maloperation onextreme load throwoffs have been observed.

V. CURRENT DIFFERENTIAL PROTECTION INPHASE COORDINATES

Positive sequence network is excited by both ground andphase faults. Hence, in principle, current differential protection


Fig. 6. Equivalent- model of three phase transposed transmission line [24].

scheme using a positive sequence network representation canalone detect all possible faults. However, sensitivity using posi-tive sequence component alone will vary with the type of fault.It will be maximum for a bolted three-phase (LLLG) fault. Incontrast, for a single line to ground fault, the sensitivity forground fault detection will be reduced by a factor of three ap-proximately.4 This motivates that either all the sequence net-works (positive, negative, zero) be used for decision making,or computation in phase co-ordinates should be employed. Weprefer the phase domain approach because of its simplicity andaccuracy.

Let us consider the equivalent- model of a three phase trans-posed transmission line as shown in Fig. 6.

Let and be the self and mutual series impedance of theline. For a transposed line, they can be easily computed from thesequence data as follows:

(19)

(20)where and are the positive, negative and zero se-quence impedance of the transmission line. Similarly, and

are self and mutual shunt susceptance of the transmissionline. They can be computed as follows:

(21)

(22)It has to be noted that, usually, will be negative as .Now from Fig. 6, the line current equation at bus in phasecoordinates can be expressed as follows:

(23)4Note that for LLLG fault, and for LG fault .

Indices , and represent the respective phases. Similarly, theline current equation at bus can be expressed as follows:

(24)Thus, we conclude that there is no fault on the line if

(25)(26)(27)

In practice, each phase tripping logic can be set using the pro-cedure described in Section IV.

Remark 1: The currents phasors and can be com-puted from GPS-synchronized measurements using (23) and(24). Total twelve GPS-synchronized measurements are re-quired, three currents and three voltages at each end. Phasorsare computed from most recent samples by recursive discreteFourier transform (DFT) [21] or phasorlets [12]. An alternativeto estimation of currents and in the phase domainwould be computation in the time domain. However, phasorapproach has been preferred because it avoids numericaldifferentiation.

Remark 2: There is no possibility of inadvertent tripping ofthe transmission line due to line charging current. This is be-cause the discriminant function value will be zeroeven during line charging.

VI. RELAYING ALGORITHMWe now propose following algorithm for sensitive and secure

current differential protection scheme.

A. At Node i1) Input line parameters, relay settings ( and

), sampling frequency and trip value forcounter .

2) Set 0.3) Acquire latest GPS-synchronized time-tagged samples

and where

Time t indicates an instant corresponding to latest sampleand , , and designate the three phases.

4) Update phasors5 and .5) Compute by using (23).6) Acquire the latest phasors, from the other end.

Note that due to communication latency .7) For instant , compute and by using (18) and

(8).5Phasors can be updated using full-cycle recursive DFT, half-cycle recursive

DFT, or phasorlet.


Fig. 7. GPS-synchronized current differential protection scheme for series-compensated line (series capacitor at end).

Fig. 8. GPS-synchronized current differential protection scheme for the series-compensated line (series capacitor at mid point).

8) Check if:OR .

If TRUE, then .else, if .

9) If issue the trip decision.Else, go back to step 3.

Similar algorithm is also applied at node the .

VII. CURRENT DIFFERENTIAL PROTECTION SCHEME FORSERIES-COMPENSATED TRANSMISSION LINES

Series capacitor on a transmission line can be installed at ei-ther end or at the midpoint. If a line is compensated at its ter-minals, then the scheme described in the previous section canbe applied in toto using line connected GPS-synchronized linecurrent and bus voltage measurements (see Fig. 7). However,if midpoint compensation is used, then the basic scheme usingsequence network representation (proposed in the Section II)should be modified as follows.

Fig. 8 shows a line with series compensation at midpoint. Theline sections of either side of series compensation are accuratelymodelled by the equivalent model of transmission line.6

6Each equivalent model corresponds to half of the line length of uncom-pensated line. Notice that with equivalent , where correspondsto half the line length.

From the bus voltage and line current measurements at bus, we estimate current in the series capacitor-MOV combination

as follows:(28)(29)(30)

Similarly, current can be estimated by the followingequations:

(31)(32)

(33)

If there is no fault on the line, then we have

However, if there is a fault on the line, then discriminant func-tion will not be zero.

Remark 3: The extension of the aforementioned scheme inphase coordinates is straightforward. For the simplicity of il-lustration, we have used sequence representation, but all calcu-lations are carried out in phase coordinates. The method can beeasily adapted even if the series compensation is not at the centerof the line. Similarly, the scheme can be extended for the pro-tection of a multiterminal line.

VIII. CASE STUDIES

To evaluate the performance of the proposed scheme, the fol-lowing methodology has been used.

1) Simulate power system response to disturbances (e.g.,faults using Electromagnetic Transient Program (EMTP)simulations. ATP [24] software has been used forsimulations.

2) Samples obtained from the EMTP simulation are fed to aMATLAB program which implements the proposed differ-ential protection scheme. Full-cycle recursive DFT, half-cycle recursive DFT, and phasorlets algorithms are used toestimate the phasors.

3) The proposed scheme is compared and contrasted with:1) the conventional GPS-based current differential schemeof [4] and 2) a more recent method reported in [7].

We report results on a two-area, 230kV, 4generator, 10bussystem (refer to Fig. 9). Detailed generator, load and line data ona 100-MVA base are given in [25]. The two areas are connectedby three parallel ac tie lines of 220 km each.

In ATP-EMTP simulation, transmission lines are representedby Clarkes model (distributed parameters) and a detailed modelis used for representing generators. The initial values of gen-erator voltage magnitude and angles are calculated from theload-flow analysis. The proposed scheme is applied for primaryprotection of one of the tie lines between node 3 and 13.The fault location is measured from bus 3. ANSI 1200:5, classC400 CT model [26] and 250 kV:100-V CVT model [27], have


Fig. 9. Single-line diagram of a two-area, four-generator, ten-bus system.

Fig. 10. Equivalent circuit of CT used in ATP simulation mH and .

TABLE I CHARACTERISTICS OF 1200:5 ANSI CT

been used for obtaining realistic CT and CCVT response duringEMTP simulations.

In these simulations, type-98 nonlinear saturable inductormodel has been used to simulate nonlinear magnetizing reac-tance of CT. It is connected across the secondary of ideal CT(refer Fig. 10). The Type-98 model requires nonlinearcharacteristics of the CT core. To convert the excitation curvespecified by pairs, the methodology describedin [28], [29] is used. The CT model data have been given inTable I.

Since standard data do not provide hysteresis informa-tion (hysteresis loss is practically negligible), the type-98model7 was considered most suitable to simulate magnetizingimpedance.

Anti-aliasing filter [21], being analog filter has to be simu-lated through ATP. It is a two stage R-C filter with a cutoff fre-quency of 360 Hz, which is well below the minimum samplingfrequency of 1 kHz used in relaying and sufficiently above the50-Hz frequency requirement for phasor extraction.

Samples obtained from ATP-EMTP simulation correspond totime-synchronized GPS samples. The time step used for ATP-EMTP simulations is 20 s. However, the relaying system dataacquisition rate is set to 1000 Hz.

7Alternative to this was type-96 model, which requires hysteresis details.

Fig. 11. Performance of proposed current differential protection scheme on ex-ternal faults. Note that relay does not pick up as all the final operating points areinside the restrain region.

The performance of the proposed scheme can be gauged byits ability to balance the following well-known contradictionsof power systems relaying:

dependability versus security; speed versus accuracy.

We also evaluate the performance with series-compensated andmutually coupled lines.

A. Dependability Versus SecurityFirst, we consider nonadaptive setting of the relay in current

differential plane which has already been outlined in Section II.Then, we provide results with the adaptive scheme.

1) External Faults: To ascertain security, it must be ascer-tained that the differential relay does not operate for any ex-ternal fault. This verification is usually carried out on the severeexternal faults. All the four types of external shunt faults (LG,LL, LLG and LLL) are simulated on buses 3 and 13, as wellas on adjacent lines 3102 and lines 13112 at 25%, 50%, and75% length. In each case, the fault resistance is varied from 0to 100 in steps of 10 and fault inception angle is variedfrom 0 to 300 in steps of 15 . For each case, we compute

, and plot thefinal operating point (marked by in Fig. 11) on current dif-ferential plane. As the always appear in restrain region, itvalidates that the proposed relay will not trip on load or externalfault. The figure also shows that the restrain region cannot be re-duced, significantly, without compromising the relay security.

Fig. 12 shows the trajectory of phase-a operating point oncurrent differential plane for the bolted LLL fault on bus 3 (ex-ternal fault) for the fault inception angle of 270 . As the relayoperating point lies inside the restrain region, the relay does notpick up on external fault.

Similar investigations carried out with the adaptive relay set-ting show that the relay does not pick up on any external fault orlarge disturbance like load throw off etc. However, conventionalscheme of [4] tends to operate on low resistance external faulton bus 3 and 13. We conclude that proposed scheme does notpick up on external faults.

2) Internal Faults: Sensitivity of the proposed scheme canbe evaluated by its ability to detect a high impedance internalfault. Fig. 13 shows the trajectory of phase-a operating pointson current differential plane for one of the cases of LL fault on


Fig. 12. Trajectory of phase-a operating point for proposed current differentialprotection scheme on external fault (LLL bolted fault on bus 3, fault inceptionangle 270 ). Note that the relay does not pick up.

Fig. 13. Trajectory of phase-a operating point of proposed current differentialprotection scheme for internal fault (LL fault at the start of line, fault inceptionangle 270 ). Note that the relay picks up.

TABLE IISENSITIVITY FOR HIGH RESISTANCE INTERNAL FAULT

phase a-b, at the start of line for the fault inception angle of270 and a large fault resistance of 600 . Table II shows thehighest resistance fault, that can be detected by the differentialprotection schemes on line , irrespective of fault location andfault inception angle. The relays were set to provide maximumsensitivity without compromising security.

The table clearly shows that the proposed scheme enables farmore sensitive relay setting than the conventional scheme of [4].With nonadaptive setting, the relay sensitivity is similar to thatof scheme suggested in [7]. This can be explained from the factthat both the methods account for line charging contributions.However, with the proposed adaptive setting strategy of the re-strain region, we notice that sensitivity of the current differentialprotection scheme improves by a factor of about 2.5. We em-phasize that this improvement in the sensitivity using adaptivesetting strategy is not at the cost of the relay security.

Remark 4: The external system can change due to variousfactors like, sudden large change in load or generation, outage ofadjacent line, single pole tripping, non simultaneous opening ofadjacent line circuit breaker etc. Simulations have been carriedout to ascertain that the proposed current differential scheme isvery robust and does not maloperate on any of the above systemdisturbances.

Fig. 14. Effect of phasor computation algorithms on relay operating time ofphase-a for LLL fault at midpoint for the proposed current differential protectionscheme (sampling frequency is 1 kHz).

3) Line Charging: The energization of line under no loadand heavy load condition is simulated and the performance ofproposed current differential scheme is compared with otherschemes. Simulation results show that the proposed scheme isimmune to line charging current. This is because the actuatingquantity of proposed scheme, is independent of the linecharging current.

4) Effect of a Mutually Coupled Line: In principle, a currentdifferential relay should be immune to effect of mutual couplingof double circuit transmission lines. To ascertain this, lineand (refer Fig. 9) are modelled as individual continuouslytransposed double circuit lines with inter-circuit zero sequencecoupling, using distributed parameters (Clarke-2 3) model.Proposed scheme is applied to line and is tested for all fourtypes of faults on line and also on line . The fault location,fault resistance and fault inception angle is also varied. Simula-tion results confirm that proposed current differential schemetrips correctly on all internal faults and does not maloperate onany external fault.

B. Speed versus Accuracy

In the proposed GPS-synchronized current differential pro-tection scheme, the phasors are updated after every sample. Thescheme is very fast (refer to Figs. 14 and 15) even if the trip deci-sion is taken on the basis of error exceeding the threshold valueconsistently for four samples. Simulation results show that theproposed scheme is very fast and takes less than half a cycle tooperate for low resistance faults. However, it needs one to twocycles to detect the faults above 500 resistance. This is ac-ceptable with high impedance faults as the fault current level islow and CTs will not saturate.

1) Phasor Estimation Algorithms: The relay operating timedepends upon the method of phasor estimation. Fig. 14 showsthe operating time with the proposed scheme when phasors areestimated by using full-cycle recursive DFT (FCDFT), half-cycle recursive DFT (HCDFT), and phasorlet with adaptive andnonadaptive settings. The studies show that with the relay set-ting, phasorlets provide the fastest relay operation followed byhalf-cycle recursive DFT and full-cycle recursive DFT, respec-tively, for the same sampling frequency. Similar behavior is ob-served for the other two phases.


Fig. 15. Effect of sampling frequency on the relay operating time of phase-afor the LLL fault at midpoint for the proposed current differential protectionscheme when phasors are estimated by using full-cycle recursive DFT.

The results also show that relay operation is faster with theadaptive control of the restrain region irrespective of the phasorestimation algorithm. The fastest relay operation is achievedwith adaptive control of restrain region and when the phasorletalgorithm is used for phasor computation.

Remark 5: It is interesting to observe that time to trip has aninverse relationship to the magnitude of fault current.

This behavior can be explained as follows. For the sake ofsimplicity, consider that the phasor is computed by using full-cycle recursive DFT and nonadaptive methodology is used. Ittakes one cycle for the phasor computation algorithm to latch toa fault current value. Assuming a linear change in the estimate,we see that for a fixed pick-up value, larger fault currents implyfaster pick up (and vice versa).

2) Sampling Frequency: The sampling rate influences the op-erating time of the relay. Fig. 15 show the operating time of pro-posed scheme with adaptive and nonadaptive settings, for thesampling frequency of 1, 2, and 2.5 kHz using full-cycle recur-sive DFT algorithm for phasors estimation. The studies showthat, 2.5 kHz sampling rate gives fastest relay operation fol-lowed by 2 and 1 kHz, respectively. However, marginal gainsin speed reduce at higher sampling frequencies i.e., a result inconcurrence with the law of diminishing marginal utility. Sim-ilar observations have been made in the context of digital dis-tance relay in [10] and [30].

C. Performance With Series-Compensated LineApplication of proposed scheme to series-compensated trans-

mission line is discussed in Section VII. All the three tie linesbetween nodes 3 and 13 (refer Fig. 9) are compensated with30% series capacitive compensation. The MOV data (connectedacross the series capacitors) is given in [24]. The parallel combi-nation of series capacitor and MOVs are placed at the midpointof lines. The initial value of generator voltage magnitudes andangles are computed from the load flow analysis of compensatedsystem. The proposed scheme is then applied for the primaryprotection of tie line . All the four types of faults (LG, LL,LLG, and LLL) are simulated on both side of series capacitor online to test the performance of proposed scheme on internalfaults. Similar faults are simulated on bus 3 and bus 13 as wellas on lines 3102 and lines 13112 to test the performance ofproposed scheme on external faults. For every fault, fault loca-tion is varied from 0% to 100% in steps of 10%, fault resistance

TABLE IIISENSITIVITY FOR HIGH-RESISTANCE INTERNAL FAULT FOR

SERIES-COMPENSATED LINE

is increased from 0 in steps of 10 and fault inception angleis varied from 0 to 300 in steps of 15 . It is validated that therelay discriminates between internal and external fault and tripson internal fault only.

Extensive case studies are carried out to compare the sensi-tivity of proposed scheme with schemes of [4] and [7]. Table IIIshows the highest resistance fault that can be detected by the dif-ferential protection schemes on line , irrespective of fault lo-cation and fault inception angle. Note that the proposed schemedetects very high resistance LG fault. Also, the sensitivity ofproposed scheme on other types of fault is better than conven-tional scheme. Further, it is seen that with nonadaptive ver-sion of the proposed methodology, sensitivity of the proposedmethod is comparable with that of method reported in [7]. How-ever, sensitivity improves significantly when proposed adaptiveprotection methodology is used. This brings out the importanceof the suggested adaptive control of the relay restrain region.

The proposed scheme is also compared and contrasted withsegregated phase comparison scheme and distance protectionscheme in presence of current and voltage inversion. It is ob-served that distance protection scheme maloperates for series-compensated transmission lines and segregated phase compar-ison scheme fails to trip in presence of current inversion [31].However, the proposed scheme works satisfactorily.

IX. CONCLUSIONSignificant advances have been made in the current differen-

tial protection schemes for transmission-line protection. State ofthe art methods consider both: 1) modeling of shunt capacitanceof line to account for line charging effect and 2) time stampedand synchronized phasors to correctly account for relative phaseangle information. No doubt, these measures have significantlyimproved the dependability and security of the current differen-tial protection schemes.

In this paper, we have proposed enhancements to further im-prove the dependability and security of the current differentialschemes. Salient contributions of the paper are as follows.

1) Development of dynamic phasor model of a transmission-line and error analysis of current differential protectionscheme using steady-state phasor.

2) An adaptive relay setting procedure to control the area ofthe restrain region in the current differential plane. The areaof the restraining region is made a function of line current.At lower currents, restraining area is kept small. This in-creases the sensitivity of the relay. At larger currents, area


of the restraining region is increased in proportion to thecurrent. This increases the security of the relay withoutcompromising the sensitivity. Simulation studies show thatthis can improve sensitivity of the relay by at least a factorof 2.5.

3) Extension of the proposed methodology for protection ofseries compensated and multiterminal transmission lines.Simulation studies show that this can improve sensitivityof the relay by at least a factor of 2.

4) Comparative evaluation with state of the art methods.5) An in depth analysis of the following contradictions:

dependability versus security; speed versus accuracy.

We conclude that the proposed adaptive control of restrainregion together with phasorlet algorithm for phasor estimationprovides the best solution for current differential protection of(series compensated) transmission lines. It enhances sensitivityand relaying speed without compromising the security of pro-tection system.

REFERENCES[1] J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and

Applications, 3rd ed. Boca Raton, FL: CRC, 2007.[2] S. C. Sun and R. E. Ray, A current differential relay system using fiber

optics communications, IEEE Trans. Power App. Syst., vol. PAS-102,no. 2, pp. 410419, Feb. 1983.

[3] P. K. Gangadharan, T. S. Sidhu, and A. Klimek, Influence of cur-rent transformer saturation on line current differential protection algo-rithums, Inst. Eng. Technol. Proc. Gen. Transm. Distrib., vol. 1, no. 2,pp. 270277, Mar. 2007.

[4] H. Y. Li, E. P. Southern, P. A. Crossley, S. Potts, S. D. A. Pickering,B. R. J. Caunce, and G. C. Weller, A new type of differential feederprotection relay using global positioning system for data synchroniza-tion, IEEE Trans. Power Del., vol. 12, no. 3, pp. 10901099, Jul. 1997.

[5] I. Hall, P. G. Beaumont, G. P. Baber, and I. Shuto, New line currentdifferential relay using GPS synchronization, presented at the IEEEBologna Power Tech Conf., Bologna, Italy, Jun. 2003.

[6] G. Houlei, J. Shifang, and H. Jiali, Development of GPS synchroniseddigital current differential protection, in Proc. Int. Conf. Power SystemTechnology, Apr. 1998, pp. 11771182.

[7] Z. Y. Xu, Z. Q. Du, L. Ran, Y. K. Wu, Q. X. Yang, and J. L. He, Acurrent differential relay for a 1000-kV UHV transmission line, IEEETrans. Power Del., vol. 22, no. 3, pp. 13921399, Jul. 2007.

[8] J. A. Jiang, C. W. Liu, and C. S. Chen, A novel adaptive PMU-basedtransmission line relayDesign and EMTP simulation results, IEEETrans. Power Del., vol. 17, no. 4, pp. 930937, Oct. 2002.

[9] S. Sheng, K. K. Li, W. L. Chan, X. J. Zeng, and D. Xianzhong, Agent-based wide area current differential protection system, in Proc. In-dustry Applications Conf., Oct. 2005, pp. 453458.

[10] G. E. Alexander, Advancements in adaptive algorithms for securehigh-speed distance protection, presented at the 23rd Annual WesternProtective Relay Conf., Oct. 1996.

[11] M. G. Adamiak and W. Premerlani, A new approach to current dif-ferential protection for transmission lines, presented at the Protec-tive Relaying Committee Meeting, Electric Council of New England,Portsmouth, NH, Oct. 1998.

[12] J. A. de la O Serna, Phasor estimation from phasorlets, IEEE Trans.Instrum. Meas., vol. 54, no. 1, pp. 134143, Feb. 2005.

[13] R. K. Gajbhiye, B. Gopi, P. Kulkarni, and S. A. Soman, Computation-ally efficient methodology for analysis of faulted power systems withseries compensated transmission lines: A phase coordinate approach,IEEE Trans. Power Del., vol. 23, no. 2, pp. 873880, Apr. 2008.

[14] M. M. Saha, B. Kasztenny, E. Rosolowki, and J. Izykowski, Firstzone algorithn for protection of series compensated lines, IEEE Trans.Power Del., vol. 16, no. 1, pp. 200207, Apr. 2001.

[15] A. A. Girgis, A. A. Sallam, and A. K. El-Din, An adaptive protectionscheme for advanced series compensated (ASC) transmission lines,IEEE Trans. Power Del., vol. 13, no. 2, pp. 414420, Apr. 1998.

[16] T. S. Sidhu and M. Khederzadeh, Series compensated line protectionenhancement by modified pilot relaying schemes, IEEE Trans. PowerDel., vol. 21, no. 3, pp. 11911198, Jul. 2006.

[17] Series capacitor bank protection, IEEE Power System RelayingCommittee Special Publication 1998, Working Group K13.

[18] G. Michel et al., Digital communications for relay protection, [On-line]. Available: http://www.pes-psrc.org/

[19] M. G. Adamaik, A. P. Apostolov, M. M. Begovic, C. F. Henville, K.E. Martin, G. L. Michel, A. G. Phadke, and J. S. Thorp, Wide areaprotecionTechnology and infrastructures, IEEE Trans. Power Del.,vol. 21, no. 2, pp. 601609, Apr. 2006.

[20] I. H. G. Brunello, I. Voloh, and J. Fitch, Current differential relaying-coping with communications channel asymmetry, in Proc. 8th IEE Int.Conf. Developments in Power System Protection, Apr. 2004, vol. 2, pp.821824.

[21] A. G. Phadke and J. S. Thorp, Computer Relaying for Power Systems.Taunton, U.K.: Research Studies Press, 1988.

[22] M. Zhang, X. Dong, Z. Q. Bo, B. R. J. Caunce, and A. Klimek, A newcurrent differential protection scheme for two terminal transmissionlines, in Proc. Power Engineering Society General Meeting, Jun. 2007,pp. 16.

[23] P. Mattavelli, G. C. Verghese, and A. M. Stankovic, Phasor dynamicsof thyristor-controlled series capacitor systems, IEEE Trans. PowerSyst., vol. 12, no. 3, pp. 12591267, Aug. 1997.

[24] H. W. Dommel, Electromagnetic Transients Program (EMTP) TheoryBook. Portland, OR: Bonneville Power Administration, 1986.

[25] K. R. Padiyar, Power System Dynamics, 2nd ed. Hydrabad, India: BSPublications, 2002.

[26] P. V. Chawande, A high fidelity and robust closed loop active fluxcompensation scheme for mitigating CT saturation, Ph.D. dissertation,Inst. Technol.-Bombay, Bombay, India, 2007.

[27] D. Fernandes, W. L. A. Neves, and J. C. A. Vasconcelos, Couplingcapacitor voltage transformer: A model for electromagnetic transientstudies, Elect. Power Syst. Res., vol. 77, pp. 125134, Mar. 2006.

[28] W. L. A. Neves and H. W. Dommel, On modelling iron core non-linearities, IEEE Trans. Power Syst., vol. 8, no. 2, pp. 417423, May1993.

[29] D. A. Tziouvaras et al., Mathematical models for current, voltage, andcoupling capacitor voltage transformers, IEEE Trans. Power Del., vol.15, no. 1, pp. 6272, Jan. 2000.

[30] J. M. Kennedy, G. E. Alexander, and J. S. Thorp, Variable digital filterresponse time in digital distance relay. no. GER3798 [Online]. Avail-able: http://www.geindustrial.com/multilin/notes/ger3798.htm

[31] S. S. Dambhare, N. W. Kinhekar, S. A. Soman, and M. C. Chandorkar,ATP-EMTP analysis of series compensated lines for distance protec-tion scheme, in Proc. Int. Conf. Power System Protection, Banglore,India, Feb. 2007, pp. 3647.

Sanjay Dambhare received the B.E. degree in electrical engineering fromthe Visvesvaraya Regional College of Engineering, Nagpur, India, in 1989,the M.Tech degree in electrical engineering from the Indian Institute of Tech-nology, Bombay, India, in 1998, and is currently pursuing the Ph.D. degree inelectrical engineering at the Indian Institute of Technology-Bombay, Mumbai.

He is currently Associate Professor at the College of Engineering, Pune,India. His research interests include power system protection, numerical relays,and power system computation.

S. A. Soman (M07) received the B.E. degree in electrical engineering fromthe Maulana Azad College of Technology, Bhopal, India, in 1989, and the M.E.and Ph.D. degrees in electrical engineering from the Indian Institute of Science,Bangalore, India, in 1992 and 1996, respectively.

Currently, he is a Professor in the Department of Electrical Engineering,Indian Institute of Technology-Bombay, Mumbai, India. He is author of thebook Computational Methods for Large Power System Analysis: An ObjectOriented Approach. His research interests and activities include large-scalepower system analysis, deregulation, application of optimization techniques,and power system protection.

M. C. Chandorkar (M84) received the B.Tech degree in electrical engineeringfrom the Indian Institute of Technology-Bombay, Mumbai, India, in 1984, theM.Tech degree in electrical engineering from the Indian Institute of Technology-Madras, Chennai, India, in 1987, and the Ph.D. degree from the University ofWisconsin, Madison, in 1995.

He has several years of experience in the power-electronics industry in India,Europe, and the U.S. During 1996-1999, he was with ABB Corporate ResearchLtd., Baden-Daettwil, Switzerland. He is currently Professor in the Departmentof Electrical Engineering, Indian Institute of Technology-Bombay, Mumbai,India. His research interests include the application of power electronicsto power-quality improvement, power system protection, power-electronicconverters, and control of electrical drives.

Adaptive Current Differential ProtectionSchemes.pdf

Documents

Transcript of Adaptive Current Differential ProtectionSchemes.pdf