Power System Economics and Market Modeling

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1 Power System Economics and arket odeling 8: Developing an LP nalysis for a Large Case 2001 South First Street Champaign, Illinois (217) support@powerworld.com

2 LP nalysis: Outline Sample PJ study: process overview One possible step by step approach for developing LP nalysis on a large case Use of Super rea to model ISO control Unenforceable constraints ore on unenforceable constraints and other OPF challenges 8: LP Large System 2

3 Sample PJ Study 8: LP Large System 3

4 Process Overview Case Development Select area(s) of interest for study Establish the set of OPF controls: OPF, unit commitment, and GC settings Establish the set of OPF constraints: Limit onitoring settings Load cost curves for thermal generators Solve unconstrained OPF for area lambdas Set hydro dispatch to historical levels and hydro cost curves to unconstrained area lambdas Solve OPF Review results, analyze unenforceable constraints, and iterate process as necessary 8: LP Large System 4

5 Case Development Suggestions Full OPF analysis on a large case may be time consuming For extremely congested cases, there may be no solution that satisfies all constraints For meaningful results, it is recommended that the scope of analysis be limited to a region of interest such as a few control areas or a single RTO territory 8: LP Large System 5

6 Case Development Suggestions lign the part of the system to be optimized with the generator controls to remove the constraints Do not monitor elements in the part of the system not on OPF control Only place the part of the system to be studied on OPF control 8: LP Large System 6

7 Eastern.pwb Eastern Interconnect Case Load the Eastern.pwb case 3964 total generating units 143 branch thermal violations in base case Suppose we wish to model an LP market for the Eastern portion of the PJ Interconnect 11 separate control areas 593 total generating units, 407 committed generating units 37 branch thermal violations in base case 8: LP Large System 7

8 Starting Case with Overloads on High Voltage Grid Note use of Emphasis, Dynamic Formatting, and dynamicallysized pie charts on one line diagram 8: LP Large System 8

9 Case Development rea/zone Filters: show areas only OPF Controls Set areas on OPF control Set GC to YES for all generators in areas except hydro (settings stored in cost curve aux file) Limit onitoring Settings Report limits for areas only, 100 kv and above Do not monitor radial lines 8: LP Large System 9

10 Limit onitoring 8: LP Large System 10

$file 08_LP Large System\EasternCostCurvePJ.$

11 Cost Curves Load cost curves for thermal units: stored in aux file 08_LP Large System\EasternCostCurvePJ.aux 8: LP Large System 11

12 Solve Unconstrained OPF Do not enforce branch or interface constraints 8: LP Large System 12

13 Set Cost Curves for Hydro Units For each Hydro Unit (advanced filter Hydro PJ) Set offer price (Wh Price 1) equal to W arginal Cost of its bus Set GC = YES 8: LP Large System 13

14 Solve Constrained OPF Enable constraint enforcement (on Constraint Options) Solve OPF, note several unenforceable constraints 8: LP Large System 14

15 LP Contour Note high LPs on receiving end of constrained lines and areas with low reserve margin 8: LP Large System 15

16 Effect of Line Constraints ROXBURY Bus: ROXBURY (221) Nom kv: rea: PENELEC (26) Zone: PN 115KV (5) 0.99 pu KV Deg $/Wh W var V 15.9 W 10.1 var 18.8 V 0.0 W 0.0 var ID W 18.7 var V 18.6 W -1.9 var 18.7 V 0.00 W 0.00 var 87% tap V CKT 1 ROXBURY etc 233 mps 99% 86% tap LP 01GRNDP pu KV P 0.00 $/Wh Note difference in LP on each end of constrained line CKT 1 CRLISLE pu KV $/Wh mps CKT 1 SHDE GP pu KV $/Wh V CKT 1 ROXB SUB 256 CRL PKE 260 CRLISLE pu KV $/Wh 8: LP Large System 16

17 Options for Further nalysis Increase available units (and reserve margin) in areas with limited supply any generators at their max output Only 4.4% operating reserves competitive market would likely have more units committed more controls Place PJ reas on Super rea Control 8: LP Large System 17

18 Unit Commitment and Reserves ost generators in high LP area DP&L on Delmarva Peninsula are at max 8: LP Large System 18

19 PJ Super rea Hydro Price = $57 8: LP Large System 19

20 Options for Further nalysis Check sensitivities on unenforceable constraints Optionally ignore or raise limits, change unit commitment, or include demand response (curtailable load) Some unenforceable constraints may be unavoidable due to load pockets Incorporate contingencies with Contingent Interfaces (flowgates) Change cost curves (e.g. model a 10% increase in fuel cost) 8: LP Large System 20

21 Unenforceable Constraints Examine LP Basis atrix Run multiple element TLR on overloaded lines to understand relationship between flows and generator and load values 8: LP Large System 21

22 LP Basis atrix arginal controller sensitivities have very low absolute value suggests presence of load pockets The sensitivity vector of each control has a mix of signs adjusting the control to relieve one constraint makes another worse 8: LP Large System 22

ultiple Element TLR TLR on overloaded lines with Super rea as buyer Negative values on decommitted generators indicate units that may relieve

23 ultiple Element TLR TLR on overloaded lines with Super rea as buyer Negative values on decommitted generators indicate units that may relieve congestion if committed Positive values on committed generators (especially those at in W) indicate that de committing may help 8: LP Large System 23

ultiple Element TLR dd ETLR field to generator display and sort Note how committed units with most negative ETLR are generally maxed out Try committing more

24 ultiple Element TLR dd ETLR field to generator display and sort Note how committed units with most negative ETLR are generally maxed out Try committing more units with negative ETLR or those with highest product of ETLR and ax W (custom expression TLR Potential ) Solve power flow, then Resolve OPF 8: LP Large System 24

Demand Response Loads may have benefit functions, allowing them to respond to price signals in the OPF Load the aux file EasternLoadBenefitodels.

25 Demand Response Loads may have benefit functions, allowing them to respond to price signals in the OPF Load the aux file EasternLoadBenefitodels.aux: includes benefit functions for 156 loads that impact unenforceable constraints Enable load controls in OPF Options and Results and re solve OPF 8: LP Large System 25

26 Demand Response OPF Load Records display or Difference Flows may be used to identify curtailed loads and marginal benefit Unenforceable constraints due to load pockets are relieved 8: LP Large System 26

27 Price Contour with Demand Response 8: LP Large System 27

28 Incorporate Contingencies with Flowgates Load EasternContingentInterfaces.aux Each flowgate interface includes a monitored element and a contingent element ake sure Contingent Interface Elements are Enforced in OPF (Simulator Options > General tab or OPF > Interfaces Display) Re solve OPF 8: LP Large System 28

29 ore on OPF Challenges 8: LP Large System 29

30 OPF Formulation and Solution ore on Unenforceable Constraints Radial Elements var loops in C power flow Unusual modeling parameters Insufficient Reserves: not enough controls to satisfy area CE constraint Too uch Power Transfer 8: LP Large System 30

31 Eastern2.pwb nalysis of Unenforceable Constraints Example: Load Eastern2.pwb (has cost info) Choose dd Ons ribbon tab Primal LP We end up with 46 unenforceable constraints Of these many seem to be caused by radial Change Limit onitoring Settings to Ignore Radial Lines and Buses Radial Bus is connected to the system by only one transmission line Radial Line is a line connected to a radial bus. Choosing this reduces the unenforceable list to 30 constraints. 8: LP Large System 31

32 Unenforceable Constraints If you look at the W and Var flows on these lines you ll find that many have VERY large Var flows dd Columns for ax W and ax Var on dd Ons ribbon tab OPF Case Info OPF Lines and Transformers If you look through the case, you ll find many very strange LTC tap ratio settings lso some are due to phase shifters being in series with an overloaded branch 8: LP Large System 32

33 Reset LTC Taps Set all transformers on LTC control to a tap ratio of 1.00 UX File: 08_LP Large System\Eastern2ChangeTransformers.aux Re solve power flow, then OPF ay also examine Circulating var Flows Tools > Connections > Find Circulating W or var Flows Check relative tap ratios in Flow Cycles with high Loss var Reduction 8: LP Large System 33

34 Unenforceable Constraints This results in a reduced list of 20 unenforceable constraints 8: LP Large System 34

35 Phase Shifting Transformers Phase Shifters have three control options None leave at a fixed angle Power Flow llow the power flow solution to dispatch according to the W setpoints of the controller OPF llow the OPF s linear program to dispatch the transformer for a more global optimization OPF phase shifter control is often necessary if load is varied with the time step simulation, unless appropriate phase shifter control settings are known for each load level 8: LP Large System 35

36 Use Caution with Phase Shifter OPF Control Phase shifter setpoints are often important for stability The setpoints may vary with load or seasonal generation pattern Options to consider: ignore V/mp limit enforcement where obvious conflicts occur between limit and phase shifter setpoints (e.g. overloaded line in series with phase shifter) allow only a few phase shifters to operate on OPF control where it is known that stability margins are sufficient choose to Enforce W Regulation Limits in OPF (branch field for phase shifters) tighten the angle limits of phase shifters to limit range of OPF dispatch 8: LP Large System 36

37 Conflict between Phase Shifter Setpoint and Line Limits WP PH.S1 Bus: WP PH.S1 (6372) Nom kv: rea: BGE (32) Zone: 32 (32) 1.01 pu KV Deg $/Wh Phase shifter setpoint is W, but line limits are <=45 V; ignore these lines Limts 8: LP Large System 161% V 64.8 W 11.9 var 65.9 V V CKT 1 GOULDST pu KV NEWGT NEWGT WP PH.S GOULDST 5906 GOULDST 6596 GOULDST tap 146% V 64.8 W var 65.9 V V CKT 1 WESTPORT pu KV WSPT WSPT WESTPORT tap 0.00 W 0.00 var 37

38 Closer Look Look more closely at the majority of the remaining unenforceable constraints Continues to show a large number of under radial elements which should probably just be ignored handful of elements require greater study Draw a oneline diagram to represent this part of the system You will start to see what the problem is Changes described in following slides may be automatically loaded with 08_LP Large System\Eastern2onitor Changes.aux 8: LP Large System 38

39 Example: Internal Shawville var 360 P-BURG $/Wh 0.99 pu W P-BURG var $/Wh 0.95 pu V var 425 PHILIPSB $/Wh 0.97 pu 436 SHWVILL $/Wh 0.98 pu 0.98 pu $/Wh V 0.97 pu 434 V SHWVILL 435 SHWVILL V $/Wh 0.96 pu SHWVILL $/Wh 1.05 pu V $/Wh pu TYRONEN 0.0 W 0.0 var V 18.7 W V 7.5 var 99% V 368 SHWVILL $/Wh 1.04 pu 151% 101% 15.1 W var V V V V 50.2 W 21.0 var V $/Wh pu DER 15.1 W 8.3 var 257 SHWVILL $/Wh V 0.99 pu 431 SHWVILL V $/Wh 0.94 pu V V 98% 17.5 W 0.0 var 15.6 W 8.5 var 152% 100% V 419 SHWVILL $/Wh 1.01 pu 423 SHWVILL $/Wh 428 SHWVILL $/Wh 1.00 pu 372 SHWVILL $/Wh 1.08 pu V V $/Wh 0.97 pu V V V V $/Wh V pu ROCKTON 50.1 W 17.1 var 424 SHWVILL 235 DER 0.3 W 0.0 var $/Wh 0.96 pu $/Wh 1.00 pu V $/Wh 0.97 pu 465 WESTOVER $/Wh 0.98 pu 300 ROCK T 421 DUBOIS Rest of the System Four of the stepup transformers experience high loadings. We could choose to ignore these limits. The lines from and also experience high loadings because the generators are all at their low limits and can not back down far enought to remove these problems. We could turn off generators at buses 431 and 424 to fix this. 8: LP Large System 4.2 W 5.5 var W 20.3 var W -9.1 var 39

40 V V V V V V V V V V V V Example: Internal erck $/Wh pu 4212 NWLES 82.2 W 25.2 var 82.2 W 14.1 var 23.2 W 9.0 var 90.5 var 99% V 4217 N WLES $/Wh pu 4216 NWLES $/Wh pu 82.2 W 25.2 var 20.9 var 4214 NWLES $/Wh pu 113% V 4215 NWLES $/Wh pu line has a large impedance of 0.15 compared to the lines , , which have impedances of $/Wh L pu $/Wh pu 3.9 W 1.6 var 4195 ERCK $/Wh pu 4154 L This means that will NEVER have any flow on it. Thus the line is essentially radial. 8: LP Large System ERCK ERCK $/Wh $/Wh pu pu 0.0 W 0.0 var 1.4 W -1.5 var 40

41 Siegfried Nazareth Limits Wh 3061 EPLERT V $/Wh pu SIEGFRIE V V % V V V $/Wh pu KEY C 2 pu V V var $/Wh 3376 EPLERT W W var var V 3195 RROWHE $/Wh 8: LP Large System 3403 V 1.01 pu PL T var V V $/Wh var 1.01 pu ECKESVI V V V var 1.70 var 56733$/Wh V 9.63 var V $/Wh 0.96 pu PL T $/Wh 1.01 pu var 64.0 W 24.3 var 64.0 W 24.3 var 64.0 W 24.3 var 3408 SIEGFRIE V $/Wh W 10.8 var 0.98 pu KEY C W 24.3 var 99% Removing the negative loads at NZRETH and an equivalent amount of positive load at SIEGFRIE relieves the otherwise difficult overloads on the branches between them. V 106% V -32 W 99% V var -32 W $/Wh $/Wh NZRETH 1.03 pu CH HL 99% V V $/Wh $/Wh p 1.03 pu CH HL T $/Wh pu LSTR T pu LSTR T2 V V V V V $/W 1.03 pu V $/ 1.03 pu 41

42 fter these changes we remove all unenforceable Constraints Still some very high cost constraints remain BIRDBORO PINE LNE = $753/Vhr 8: LP Large System 42

43 V V var V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V Birdboro Pine Lane Yellow Region forms a load pocket for two large loads 85.3 W W The 69 kv lines feeding this region have high loadings 8: LP Large System $/Wh $/Wh 1.06 pu 1156 NBOYERTO 1590 NBOYERTO $/Wh W TITUS 0.0 var $/Wh 35.9 W $/Wh 0.0 var $/Wh pu TITUS $/Wh 1611 W.BOYTWN $/Wh 1582 LORNE 1593 V PINE LNE 0.0 W 0.0 var 1563 BIRDFERO 1596 RNGROCKS 1567 CONTY LN 1600 SREDING 1553 RORCST 1.06 pu 32.3 W 0.0 var $/Wh 1730 TITUS $/Wh $/Wh 1607 TITUS 1608 TITUS $/Wh 85.3 W var 100% V 100% $/Wh 1606 TITUS 1562 BIRDBORO $/Wh 1575 K.B.I BRTO $/Wh $/Wh 1164 SREDING $/Wh $/Wh $/Wh 1.01 pu $/Wh $/Wh 1.00 pu $/Wh $/Wh 94% V 1566 CLOUSER $/Wh 1.01 pu 4.0 W 0.0 var 0.0 W 0.0 var $/Wh 1.00 pu $/Wh 1.00 pu 1570 E.TOPTON 1574 GLENSIDE 1573 FRIEDNBG 4.2 W 0.0 var $/Wh 95% V $/Wh 1.01 pu 1569 E PENN 1576 KUTZTOWN 1729 SREDING 1565 CRSONI 1585 C-KN GP $/Wh $/Wh 1.00 pu $/Wh $/Wh $/Wh $/Wh 1.01 pu 1609 CORSTK T 1571 EXIDE $/Wh 1555 BLDY 1572 FLYING H 1610 U.CORSTK 1.00 pu 1583 LYNNVILE $/Wh $/Wh $/Wh W $/Wh 1.01 pu 1554 T&T 1599 S.HBRG 1584 LYONS $/Wh V var 1612 W.RDG $/Wh $/Wh $/Wh $/Wh $/Wh 0.96 pu $/Wh 1.01 pu $/Wh $/Wh 0.96 pu $/Wh 0.96 pu $/Wh 83% 1551 DSTWN 1564 CR TECH 1154 LYONS 1568 DN 1598 S.HBRG 1715 HILL RD LINC SPG VL 1578 LEESPORT HILL RD 1717 PNTHER 1704 PNTHER 63.4 W 0.0 var $/Wh $/Wh $/Wh 97% V 1580 LINC LEESPORT $/Wh $/Wh 1.01 pu $/Wh 1.01 pu $/Wh $/Wh 1592 OUTR ST 1586 G IND 1587 G IND T 1557 BERK $/Wh 1561 BERNVILL $/Wh 1588 OSELE 1.01 pu 1581 LINCOLN 1603 SION TP $/Wh 1560 BERN CH $/Wh $/Wh 16.4 W -3.3 var 1589 UHLENBG $/Wh $/Wh 1602 SION $/Wh 1605 ST PETRS $/Wh 1558 BERK $/Wh 1597 ROSEDLE 1595 RIVRVIEW $/Wh 1559 BERKLEY $/Wh 1.06 pu $/Wh 1.01 pu 12.6 W 5.0 var 1591 NTEPLE 1552 LTN CT $/Wh 1.01 pu 1159 NTEPLE W var 43

V V V V V V V V V V V Contour of Prices around Birdboro Pine Lane Load Pocket These prices could be reasonable. 15.47 $/Wh 1.06 pu 15.59 $/Wh 1.06 pu 1731 TITUS 35.9 W 16.33 $/Wh 0.0 var 15.

44 V V var V V V V 100% V V V V 100% V V V V V V V V V V V V V V V V V V V V 94% V V V V V V 95% V V V V V V V V V V V V V V V V V 83% V V V V V V V V V V V V V V 97% V V V V V V V V V V V V V V V V V V V V V V V V V V V V V Contour of Prices around Birdboro Pine Lane Load Pocket These prices could be reasonable $/Wh 1.06 pu $/Wh 1.06 pu 1731 TITUS 35.9 W $/Wh 0.0 var $/Wh pu TITUS 32.3 W 0.0 var $/Wh $/Wh 32.3 W 0.0 var 1730 TITUS $/Wh $/Wh 1607 TITUS 1608 TITUS $/Wh 1582 LORNE 1606 TITUS 1562 BIRDBORO $/Wh 1563 BIRDFERO 1164 SREDING $/Wh 1.01 pu 1600 SREDING 1553 RORCST $/Wh $/Wh $/Wh $/Wh 4.0 W 0.0 var 4.2 W 0.0 var 1574 GLENSIDE 1729 SREDING $/Wh $/Wh $/Wh 1610 U.CORSTK 1609 CORSTK T 1572 FLYING H $/Wh $/Wh $/Wh 1.01 pu $/Wh 1.01 pu 1612 W.RDG $/Wh 1551 DSTWN 1568 DN 1564 CR TECH 1579 LINC $/Wh 1580 LINC $/Wh 1.01 pu 16.4 W -3.3 var 1581 LINCOLN $/Wh 12.6 W 5.0 var 1595 RIVRVIEW PINE LNE 0.0 W 0.0 var $/Wh 1565 CRSONI $/Wh 1592 OUTR ST $/Wh 1156 NBOYERTO 1611 W.BOYTWN RNGROCKS $/Wh 1585 C-KN GP $/Wh $/Wh 1586 G IND $/Wh $/Wh 1567 CONTY LN $/Wh $/Wh 1604 SPG VL $/Wh 1587 G IND T UHLENBG $/Wh 1.01 pu NTEPLE 1590 NBOYERTO $/Wh 1575 K.B.I $/Wh 1571 EXIDE $/Wh 1554 T&T $/Wh $/Wh 1597 ROSEDLE $/Wh W var 1591 NTEPLE W var FRIEDNBG $/Wh 1.01 pu 0.0 W 0.0 var 1599 S.HBRG $/Wh 1578 LEESPORT $/Wh 1577 LEESPORT 1557 BERK $/Wh 1603 SION TP $/Wh 1602 SION $/Wh 1566 CLOUSER $/Wh $/Wh 1.01 pu $/Wh S.HBRG BERK $/Wh $/Wh $/Wh 1.01 pu 1583 LYNNVILE $/Wh $/Wh 1715 HILL RD BERNVILL $/Wh 1560 BERN CH $/Wh pu E PENN 0.96 pu HILL RD BERKLEY $/Wh 1556 BRTO $/Wh 1.00 pu 1555 BLDY $/Wh 0.96 pu $/Wh 0.96 pu 1717 PNTHER 1704 PNTHER $/Wh 1.06 pu $/Wh pu LTN CT 63.4 W 0.0 var $/Wh 1.00 pu 1576 KUTZTOWN W var $/Wh 1.01 pu 1588 OSELE $/Wh 1.01 pu 1605 ST PETRS $/Wh 1.00 pu 1570 E.TOPTON $/Wh 1.00 pu 1584 LYONS $/Wh 1154 LYONS 8: LP Large System 44

45 Unenforceable Constraints Summary Look for radial systems and load pockets Look for generators or phase shifters which can relieve problems Give the OPF more controls to FIX the problems Look for constraints which don t make sense Radial lines serving load Radial transformers/lines leaving generators Use your judgment to setup a reasonable case Realize that some unenforceable constraints are inevitable at first 8: LP Large System 45

$pwb and 08 LP Large System\aux1000aster.$ aux, then solve LP OPF In this example, rea 28 (JCP&L) does not

46 Eastern.pwb Insufficient Reserves Load Eastern.pwb and 08 LP Large System\aux1000aster.aux, then solve LP OPF In this example, rea 28 (JCP&L) does not have enough GCable generation essage Log: Insufficient controls to enforce area constraint 8: LP Large System 46

47 Insufficient Reserves: Tips Examine Generator records or rea field Gen W GC Range Up To resolve Commit more generation ake more generation GCable, or designate some units as OPF Fast Start Increase imports, or make rea part of a Super rea Decrease load, or make load dispatchable 8: LP Large System 47

48 08EasternWidereaarket.pwb Too uch Power Transfer In the OPF solution, a linear program (LP) iterates with the non linear power flow to achieve convergence of the entire solution If an C OPF is performed over a very large rea (or Super rea), the LP may dispatch generators in a manner that exceeds voltage stability margins WECC cases may be especially susceptible 8: LP Large System 48

49 Too uch Power Transfer Example: Northern and a Southern section of the eastern interconnection are modeled as one ISO market (Super rea) North: includes eastern PJ, EP, First Energy South: includes TV, Southern Company, Entergy ssume generation much less expensive in the south, so LP OPF will initially try to increase the transfer from south to north OPF may exceed stability margin of power flow Load 08EasternWidereaarket.pwb, then solve LP OPF 8: LP Large System 49

50 Too uch Power Transfer Excerpt from essage Log LP attempts massive power transfer Game Over! 8: LP Large System 50

51 Too uch Power Transfer: Tips Tighten W Limits on generators with unrealistic limits (e.g. ax W = 9999): in this example, GC is set to NO for such units Place less of the system on OPF control Use interface limits Break a large area (or super area) into two or more smaller areas; use OPF dispatchable transactions between the smaller areas anually move generation in the direction of the LP transfer, resolve power flow, restart OPF Use DC Power Flow 8: LP Large System 51

52 OPF Dispatchable Transactions Example: Break WG super area into Northern and Southern super areas Reopen 08EasternWidereaarket.pwb and load 08_LP Large System\EasternNorthSouthSuperreas.aux dd a new transaction between a Northern area (e.g. EP) and a Southern area (e.g. TV) Set W limits on the new transaction and make it Dispatchable in OPF If OPF and power flow solve, try increasing the limits of the new transaction stop when the power flow will not converge 8: LP Large System 52

53 OPF Dispatchable Transactions South North transaction limited to 500 W beyond base case transfer OPF determines optimal transaction. If transaction is non binding at the solution, then areas are acting as a single super area. 8: LP Large System 53

54 anually ove Generation Reopen 08EasternWidereaarket.pwb Solve a Single Outer Loop of the OPF Look at OPF controls following the failure ove generation in direction of transfer (e.g. 10% of the transfer) ttempt to resolve the OPF Often additional transmission constraints will become binding before the full transfer is made OPF will know to move in a different direction 8: LP Large System 54

55 anually ove Generation: OPF Controls 8: LP Large System 55

56 Use a spreadsheet to step the transfer (Generatordjust.xls) Set GenW = Orig. Value + Delta Value * Percent ove 1. Copy OPF controls to the spreadsheet 2. Paste Gen Records back into Simulator, solve power flow, and restart LP OPF anually ove Generation: Spreadsheet 8: LP Large System 56

Techniques for Conditioning Hard-to-Solve Cases. Overview

Techniques for Conditioning Hard-to-Solve Cases James Weber Director of Operations PowerWorld Corporation weber@powerworld.com Phone: 217 384 6330 ext 13 1 Overview Starting with public text-file data