E-TRAN - CiteSeer
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1. sparse methods to collapse Yfull from an NxN ma trix to a KxK matrix called Yreduced The admittance matrix of all branches and devices entirely contained in the K kept busses is created let this be Ykept also a KxK matrix The admittance matrix of the network equivalent called Yeq is the difference between the reduced Y matrix Yreduced and the kept Y matrix Ykept ie Yeq Yreduced Ykept The elements of Yeq are realized as a circuit with RLC elements and mutually coupled elements The off diagonal entries are represented by either a PI section or a transformer if the bus voltages at each end of the branch are different and the diagonal entries are represented with a voltage source to ground The voltage source magnitude and phase of the di agonal elements in the network equivalent are automatically calculated to give the correct load flow in the kept network The resulting multi port network equivalent will give the correct loadflow solution under normal operating condi tions but will also give the correct faulted and open circuit solutions Single port network equivalents are commonly used and although they generate the correct steady state loadflow solutions they will not provide the correct faulted or open circuit solutions as they do not represent the off diagonal entries of the Yeq matrix The translation pro gram includes an optional off diagonal Ylimit parameter which can be used to ignor2. A 160 BUS SYSTEM REDUCED TO 15 BUSSES OF INTEREST WITH A NETWORK EQUIVALENT This example shows a portion of a small network at a 345 kV bus The circuit is loaded in PSCAD V4 and was directly generated by the translation program from a PSS E raw input file The program directly translated only 2 busses away from the main bus which resulted in a 15 bus circuit see Fig ure 4 and formed a network equivalent of the remaining circuit The equivalent network on a sub page uses trans former branches for the off diagonal branches since the interface busses are at different voltage levels In this case the entire circuit was only 160 busses so it could have easily been translated and run in PSCAD with out the need for any network equivalent The example cases shown in this paper were derived from relatively small loadflow cases in order for demon stration purposes but very large up to 50 000 bus load flow cases have been translated successfully by the pro gram The only known limitation on the network size limit is determined by the computer memory available VIII CONCLUSIONS This paper describes a program which translates popular loadflow data which does not contain graphical informa tion directly into an electromagnetic transients program The process includes the formulation of multi port network equivalents the auto routing of the circuit so as to gener ate a graphical view of the network and the ability to sub stit
3. FACTS devices HVDC Links etc will easily startup during this period since the AC network short circuit ratio is ar tificially high ie the system impedance is lower Initialize the electrical circuitry in the machine model and switch to a machine model which is run ning with its internal rotor angle fixed ie mechani cal dynamics are still bypassed Initialize all mechanical and external control func tions such as the torque governor field volt age exciter multi mass models etc and fully re lease all machine models The system is now fully initialized and ready for faults disturbances It is suggested that a Snapshot be taken as used in PSCAD EMTDC so as to save these initial conditions in an external input file This allows multiple runs and subsequent studies to be performed quickly without having to re run the initialization process The loadflow program calculates the steady state bus magnitude and angles as well as the real and reactive power in all generator machines The translation program uses this information inside its calculations and in the formation of network equivalents and generates the EMT simulation files a psc file for PSCAD EMTDC By de fault it generates the circuit with simple voltage sources for each machine but if a machine is in the Substitution Li brary it will directly set the initial conditions in the ma chine model or any model as per the procedure outlined One minor fla
4. this example the program directly translated only 1 bus away which resulted in a 15 bus circuit from the HVDC bus and formed a network equivalent of the remaining circuit Figure 3 shows the equivalent network on a sub page The voltage magnitude and phase angle of the equivalent sources are automatically calculated to match the power flow of the solved loadflow Note the multi port equivalent is valid for steady state as well as for open short circuit conditions The isolated diagonal sources at the bottom of Figure 3 indicate isolated portions of the network which feed radially from busses in the kept network and are not connected to the bulk of the network equivalent In any network equivalent it would be wise to look at the diagonal or off diagonal impedances in the network equivalent and to include more busses in the direct transla tion if the impedances are small the small impedances generally indicate strong coupling from nearby transmis sion lines or generators It is also recommended that the transmission lines and devices close to the area of study be represented with detailed frequency dependent and or non linear device models This substitution can be automated using the Substitution Library see Section II It often requires experience and judgment on the part of the user to determine how much of the system should be represented in detail and at what point an equivalent can be inserted VII EXAMPLE 2 TRANSLATION OF
5. International Conference on Power Systems Transients IPST 2003 in New Orleans USA 1 E TRAN Translation of Loadflow Stability Data into Electromagnetic Transients Programs Garth Irwin1 and Dennis Woodford1 1 Electranix Corporation 107 865 Waverley Street Winnipeg Manitoba R3T 5P4 Canada e mail gdi electranix com daw electranix com www http www electranix com Abstract This paper outlines a new program called E TRAN which bridges the gap between loadflow stability programs and electro magnetic transients programs It is essentially a translation program for power systems but includes many features to avoid manual calculations such as the automated formation of multi port network equivalents initialization of machines sources in large systems auto routing to generate graphical views of the network data sanity checks The program currently supports the PSS E raw loadflow input format and is also partially capable of translating ATP EMTP data files into PSCAD EMTDC V3 or V4 Keywords E TRAN loadflow transient stability electromag netic transients network equivalents PSCAD EMTDC I INTRODUCTION Most utilities use loadflow and transient stability pro grams to study the fundamental frequency behaviour and interactions in power systems They often maintain large databases of their network data suitable for such studies Power systems are becoming more complex and EMT Electromagnetic T
6. adflow case say an inductor of 0 001 pu In order for power to flow through this branch a very small phase shift is required Xl P sin 2 1 2 1 V V The problem is that the PSS E raw format 2 outputs the bus phase angles i to a limited number of decimal points so it is possible that the bus angles across a low impedance branch read from the data file are identical so no power flows yet the internal loadflow solution will show power flowing This will show up as opposing power mis matches at the busses at each end of the branch but will not affect the remainder of the solution This problem is solved for non transformer branches using the zero impedance branch feature in PSS E ie make R 0 0 and the L less than or equal to 0 0001 which will make the branch impedance 0 0 and effectively collapse the two busses together into a single bus The same zero impedance branch logic and collapsing methods are fol lowed in the translation program allowing any number of International Conference on Power Systems Transients IPST 2003 in New Orleans USA 4 series parallel and loop connections of ideal branches and transferred to the PSCAD EMTDC models Users should be aware of the type of measurements they are using to compare the EMT results to the loadflow re sults EMT simulations can contain harmonic distortions dc offsets particularly near DC links or saturated trans formers etc whi
7. atically used in new cases instead of the basic RXB data available in loadflow files The Substitution Library can also contain custom written as well as any circuit contained in multi level page compo nents For example a detailed DC link or SVC model can be maintained in the library complete with multi level sub pages with exact control models If the Substitution Li brary contains a machine E TRAN will replace the re quired initial condition information in the machine based on the loadflow solution thus ensuring a bumpless initiali zation of the electrical and mechanical machine dynamics III GENERATION OF MULTI PORT NETWORK EQUIVALENTS When the input loadflow circuit is too large to run in an EMT simulation program the translation program can automatically create a multi port network equivalent The network equivalent is derived by a network reduction of the fundamental frequency admittance matrix Y of the entire network The following procedure is followed International Conference on Power Systems Transients IPST 2003 in New Orleans USA 3 The admittance matrix of the entire system Yfull is generated in Per Unit let there be N busses in the original loadflow file Busses to be kept ie translated in detail are iden tified let there be K busses to be kept and Yfull is re ordered so as to move the kept busses to the bot tom of the Yfull matrix A partial LDU decomposition is performed using
8. ch can affect some measurement tech niques such as RMS The program measurement compo nents in the Runtime Library utilize a very fast recursive DFT algorithm which extracts the positive sequence fun damental frequency quantities so measurements made with using these components should match the loadflow results Users should also be sure that the fundamental frequency behaviour of any device used in the Substitution Library which the program will output to the case it translates in stead of using the simple RXB translated model data see Section II should match that used in the loadflow For example if a user puts a detailed frequency dependent transmission line model in the Substitution Library then the program will output this data instead of the simple RXB Bergeron traveling wave model To ensure the loadflow matches 100 the user should use the Line Constants output from the frequency depend ent line model in the EMT program to obtain the positive sequence RXB values which are for an incremental unit length update re solve the loadflow file and finally re run the translation program For very long transmission lines correction factors should be applied before using the data in the loadflow program the E TRAN User s Manual documents suitable long line correction formula V INITIALIZATION OF LARGE MULTI MACHINE SYSTEMS IN EMT PROGRAMS EMT simulations with multiple machines or sources must have the system properly i
9. ches to ground are drawn A new trunk is drawn working in reverse from existing nodes and the procedure is followed recursively until all nodes and branches in a given page have been drawn The resulting drawings are guaranteed to be drawn cor rectly regardless of the complexity of the interconnections If the circuit to be drawn in PSCAD will not fit on a page the program will use the voltage levels to separate the drawings onto sub pages and sub sub pages etc If a single voltage level will not fit on the page it will still cre ate sub pages and sub sub pages etc based on geometri cal positioning of the busses Entire circuits of more than 50 000 busses have been drawn and translated in their en tirety The resulting output file can be directly loaded in PSCAD V3 as a single phase circuit or in PSCAD V4 as a three phase SLD Single Line Diagram E TRAN Runtime Library for PSCAD A library of components for PSCAD has been written which contains all devices required in the loadflow data file Transmission Lines both PI sections and Bergeron traveling wave models Transformers and Phase Shifters including the ex traction of modified leakage reactances from im pedance correction tables Loads Constant PQ Current Impedance and RLC Switched Shunt Devices Zero Impedance Branches DC Links multi terminal links not yet supported Each model in the library accepts the same data format as is u
10. e extremely weak off diagonal connections this may be useful in real time applications where sparsity in the network equivalent is desirable If the off diagonal Ylimit parameter is set very large then multiple single port equivalents will be generated without off diagonal elements Note that the network equivalent will not be of size KxK It will contain elements only where portions of the network are not fully contained in the kept network The above calculations are complicated by the presence of ideal branches ie 0 resistance inductance series ele ments and phase shifters The ideal branches are correctly handled in the program matrix calculations and it is en sured that ideal branches are entirely contained in either the network equivalent or in the kept network they cannot span from one to the other if they do a bus is added to the kept network Phase shifters are generally represented by a non symmetrical admittance matrix which is not suit able for representation by a passive network equivalent sequence circuit The phase shifters that are part of the network to be equivalenced in the program are represented with symmetrical admittance elements and the voltage source terms on the diagonals of the network equivalent are modified to provide an equivalent solution IV MATCHING THE LOADFLOW SOLUTION USING EMT PROGRAMS Loadflow programs solve the steady state fundamental frequency equations of a large power system usi
11. entified and all options set the GUI launches the solution engine The solution engine is contained in a separate DLL Dy namic Link Library so it can be imbedded in other user written programs E TRAN V1 supports the following input formats Full support for PSS E versions 23 26 raw and seq files 2 Partial support for EMTP ATP data files The following output formats are supported PSCAD V3 psc file single phase circuits only 3 PSCAD V4 psc file three phase circuits using SLD Single Line Diagram drawing 3 Additional data formats and versions are being worked on In addition to the translated output file a log file is generated which contains details of the conversion process error warning messages and network equivalent details Graphical Auto Routing The PSS E raw input file does not contain graphical co ordinate information although a draw file may be avail able for portions of the network so translation of data into PSCAD which is graphically based requires auto routing to determine the positions and coordinates of all devices The translation program performs the auto routing using formal Graph Theory mathematical algorithms The basic concept is to select a starting node and to draw in one line all series branches which connect to new nodes this is called a trunk Next all feedback branches other se ries branches connected between nodes in the same trunk and all bran
12. ng an itera tive algorithm The result is the magnitude and phase of all bus voltages as well as the PQ flow through all network branches The iterative process will continue until the sum of PQ flows at any bus called the PQ mismatch are smaller then the convergence criteria say 0 001 pu The heart of the solution is the complex system admittance ma trix Y V w Y w i w where Y w 1 0 R jwL EMT programs solve the differential equations of the power system 1 such as V t R i t L di dt The EMT solution algorithm does not require iterations but is a direct matrix solution The accuracy of the solution depends on the frequency of interest and the selected time step often 50 uSec At fundamental frequency the solu tion integration error is generally negligible since there are many samples over a fundamental frequency period so the EMT steady state solution should theoretically match very closely to the loadflow solution Circuits translated by E TRAN have been observed to match the loadflow to a very high degree of accuracy whether it is a very large system 1 000 s of busses or a small system run with a network equivalent The use of the multi port network equivalent ensures the steady state short circuit and open circuit conditions will be satisfied not possible with simple equivalent sources placed at boundary busses One potential problem occurs when a very small imped ance is used in the original lo
13. nitialized in order to avoid long run up times associated with machine inertial swings governor time constants etc The critical parameters which must be initialized are the internal angles of all ma chines and sources as well as the state variables of all con trol circuits which have long time constants such as gov ernors exciters stabilizers multi mass SSR sub synchronous resonance models etc It is not critical to fully initialize the state variables of the entire AC network as it generally contains much smaller time constants and has sufficient damping so the system can be started from 0 0 conditions and ramped up to the steady state in a very short time A complete initializa tion is possible for simple systems but a generalized con nection of devices with more complex systems with FACTS devices power electronics non linear branches etc makes this either too time consuming or un reliable A very reliable procedure that works with large and complex systems is as follows Start the EMT simulation with all voltages and cur rents at 0 0 conditions Model the machines initially as a simple ideal volt age source or one with the series impedance as de fined by ZR and ZX in the loadflow generator data whose magnitude and phase angle are determined by the loadflow program this bypasses long time constants in the electrical and mechanical compo nents of the machine Run the network up to steady state
14. ransient programs are being used more frequently for large system studies particularly when de tailed modeling of control systems and non linear devices are required EMT programs can be used for studies in volving complex interactions in systems with multiple gen erators motors wind farms HVDC links FACTS devices etc which often cannot be adequately studied by tran sient stability analysis Many utilities have the data available for their entire sys tem in loadflow program format and great effort is re quired to re enter the network data for use in EMT pro grams The authors of this paper face this problem fre quently when they help clients perform studies They de veloped the E TRAN program in order to bridge the gap between loadflow stability programs and EMT programs The translation program performs the following func tions Translates PSS E raw input files into PSCAD EMTDC V3 or V4 either entire cases or portions of a case Auto generates multi port network equivalents Auto routes non graphical bus branch load connec tion data into graphical components in PSCAD Initializes large systems with multiple ma chines source generators PSS E is a registered trademark of Power Technologies Inc PSCAD is a registered trademark of Manitoba HVDC Research Centre EMTDC is a registered trademark of Manitoba Hydro II TRANSLATION OF LOADFLOW DATA INTO AN EMT PROGRAM Most loadflow program
15. sed in the PSS E raw file ie PU impedances RXB on 100 MVA base L L Voltages etc Each model can be used with single phase circuits three phase circuits with 3 phase connections or three phase SLD circuits a single electrical array connection instead of 3 phase connections This runtime library is freely available Substitution of Detailed Device Data EMT studies often require more detailed device data including non linearities transformer saturation surge arresters power electronic devices and harmonic sources and other specialized data not required by load flow programs such as transformer winding and core ge ometries E TRAN includes support for a Substitution Library in which a user can maintain detailed model data for a given device The program uses the bus from and to numbers and the circuit identifier to uniquely identify an element to be substituted After translating a circuit it first consults the Substitution Library to see if more detailed data exists for each device and if so it will write out the detailed data instead of the basic RXB model For example a user can maintain a Substitution Library containing detailed core winding geometries and saturation data in transformers data not available in the raw input files Users can also maintain frequency dependent trans mission line data containing conductor geometry and fre quency dependent model parameters etc The detailed data will be autom
16. use or can export to the PSS E raw data input format This format includes data for busses branches generators loads switched shunt de vices dc links etc Many loadflow database files are quite large gt 50 000 busses which makes the case prohibitively large to run entirely in EMT programs In this case the user can iden tify a portion of the network to translate in detail and the translation program will automatically create a network equivalent for the remainder of the network for details see Section III below The user can identify the portion of the network to translate in detail using 5 methods Keep all busses Keep all busses in an Area Keep all busses in a Zone Keep a central bus and N busses away This op tion includes a voltage level ie keep all busses 230 kV and above and 3 busses away from my central bus Keep listed busses Figure 1 shows the Graphical User Interface GUI dia log which is used to determine which portion of a large network is to be translated in detail Figure 1 Bus Selection GUI Dialog International Conference on Power Systems Transients IPST 2003 in New Orleans USA 2 The translation program runs on any MS Windows plat form It consists of a GUI Graphical User Interface using a Wizard Style which means the user completes a form and then clicks on Next to proceed to the next set of op tions Once all input and output files have been id
17. ute detailed non linear model data suitable for EMT studies instead of simple RXB type loadflow data The resulting translated circuits in an EMT simulation are able to match the loadflow solution to a high degree of accuracy whether the entire circuit or a portion of the net work is translated and a network equivalent is used ACKNOWLEDGMENTS The authors wish to thank the Manitoba HVDC Re search Centre Inc for their assistance in providing details of the PSCAD V3 and V4 graphical data formats REFERENCES 1 H W Dommel Digital computer solution of electromagnetic transients in single and multi phase networks IEEE Trans Power App and Syst Vol PAS 88 4 pp 388 398 1969 2 PSS E Program Operation Manual 3 PSCAD User s Guide and EMTDC Theory Manual Figure 2 Example 1 A 15 Bus Circuit Generated by E TRAN from 18 000 Bus Loadflow Case International Conference on Power Systems Transients IPST 2003 in New Orleans USA 6 Figure 4 Example 2 A Small Network Concentrating on a 345 kV Bus Figure 3 Example 1 Network Equivalent Sub Page
18. w with this initialization or any method which relies on positive sequence loadflow results is that a small oscillation may arise at the point of initialization if the system has any negative sequence components of volt age or current This can occur when there are unbalanced non ideally transposed transmission lines or unbalanced loads The negative sequence component in the system results in a small second harmonic on the field torque of the machine which will show up on the machine frequency even in steady state Thus the time at which the machine is released may introduce a small offset which cannot be avoided this is generally a minor concern It should be theoretically possible to perform a three phase loadflow using phase based quantities instead of a positive sequence loadflow transfer the positive and negative sequence ini tial conditions to the machine model and initialize the me chanical electrical machine equations including the nega tive sequence effects but this has not been accomplished nor is it critical to do so in the author s opinion VI EXAMPLE 1 TRANSLATION OF A 18 000 BUS SYSTEM INTO A 15 BUS CIRCUIT NEAR A DC LINK WITH A NETWORK EQUIVALENT This example shows a very dense portion of a network near a bipolar HVDC converter station see Figure 2 The International Conference on Power Systems Transients IPST 2003 in New Orleans USA 5 original circuit was more than 18 000 busses but for
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