07 IEEE. Peronal ue of thi material i permitted. Permiion from IEEE mut be obtained for all other ue, in any current or future media, including reprinting/republihing thi material for advertiing or promotional purpoe, creating new collective work, for reale or reditribution to erver or lit, or reue of any copyrighted component of thi work in other work. DOI: 0.09/PTC.07.79809 Implementation and Validation of the Nordic Tet Sytem in DIgSILENT PowerFactory Lui David Pabón Opina André Felipe Correa Department of Power Sytem Control and Dynamic Fraunhofer IWES Kael, Germany lui.david.pabon.opina@iwe.fraunhofer.de Gutav Lammert Department of Energy Management and Power Sytem Operation Univerity of Kael Kael, Germany gutav.lammert@uni-kael.de Abtract Thi paper preent the implementation of the Nordic tet ytem in DIgSILENT PowerFactory. The implemented ytem i a variant of the Nordic network and i baed on the technical report Tet Sytem for Voltage Stability Analyi and Security Aement prepared by the IEEE Power Sytem Dynamic Performance Committee, the Power Sytem Stability Subcommittee, and the Tet Sytem for Voltage Stability and Security Aement Tak Force. Thi work deem all of the implemented controller and validate the PowerFactory implementation againt the original model preented in the aforementioned technical report. It i hown through RMS imulation that the action of load tap changer and over excitation limiter are leading force through a long-term voltage collape and therefore, their accurate modeling i crucial. Furthermore, the implemented ytem can be whether downloaded from the IEEE PES Power Sytem Dynamic Performance Committee webite or requeted directly to the author. Index Term Dynamic ecurity aement, long-term voltage tability, Nordic tet ytem, voltage collape. I. INTRODUCTION The Nordic tet ytem i a variation of the o-called Nordic ytem, which wa created in order to illutrate the voltage collape in Sweden that happened in the year 98 []. It ha been propoed by the CIGRE Tak Force 8-0- 08 in 995 [] and it i a fictional approximation of the Swedih ytem that along with other network model wa propoed in order to ae the performance of imulation tool and provide reearcher with benchmark to develop the Long-Term Dynamic field. Neverthele, the ytem i not limited to long-term phenomena, it applicability to hortterm dynamic uch a tranient tability and mall-ignal ocillatory angle tability i recognized. From it very firt appearance on, the Nordic tet ytem ha been ued not only for oftware teting purpoe but alo for other ort of dynamic tudie uch a hort-term and long-term voltage tability, power ytem retoration, ytem protection and mall-ignal tability tudie among many other. In [], it i hown through RMS imulation carried out uing PSS/E and EUROSTAG the impact of Over Excitation Limiter (OEL) and Load Tap Changer (LTC) on the long-term voltage collape. In [], the Nordic tet ytem i ued in order to invetigate power ytem retoration iue. In [], a modified verion of the Nordic ytem which include induction motor load i ued to invetigate the delayed voltage recovery after ytem fault. In [4], a coordinated decentralized emergency voltage and reactive power control to prevent long-term voltage intability i propoed and it effectivene i proven on the Nordic tet ytem. In [5], an algorithm which prevent undeirable ditance protection operation that might reult during voltage intability period i teted on the Nordic power ytem. In [6], the Nordic ytem i implified in a 5 bubar tet ytem named N5area in which the key voltage collape characteritic of the Nordic are reflected in order to tet in a le complex network o the action of load recovery and excitation current limiter are eaier to imulate. In [7], mall-ignal tability i aeed uing the Nordic. In [8], a procedure to identify the onet of long-term voltage intability from the time evolution of the ditribution voltage controlled by LTC i propoed and demontrated uing the Nordic tet ytem. Due to it particular characteritic, the Nordic tet ytem ha hown a growing demand for different kind of power ytem tability tudie. Therefore, the IEEE Power Sytem Dynamic Performance Committee provide in [9] input data for different oftware tool including RAMSES [0], PSS/E, and ANATEM. DIgSILENT PowerFactory i a widely ued commercial power ytem analyi oftware, epecially in Europe and South America. Numerou tranmiion and ditribution ytem operator a well a reearch intitute and univeritie ue thi oftware imulation tool []. In order to erve the need of academia and indutry, the new contribution of thi paper i the implementation and validation of the Nordic tet ytem in DIgSILENT PowerFactory. Furthermore, the implemented ytem can be whether downloaded from the IEEE PES Power Sytem Dynamic Performance Committee webite or requeted directly to the author. II. SYSTEM DESCRIPTION The Nordic tet ytem depicted in Fig. conit of 4 area: an equivalent implified network that ha the bigget generator and therefore erve uually a the ytem reference, the northern region with few load and more generation, a central area with more load than generation and a outhern region looely connected to the ret of the ytem. The tet network
g9 407 40 g9 0 0 g P = P 0 ( V V 0 ) α () 7 7 g8 404 406 6 6 g8 407 EQUIV. g0 0 0 40 g0 g4 0 0 4 CS g g7 g7 406 406 6 04 g5 04 SOUTH 40 40 CENTRAL 4 4044 045 5 044 NORTH g 4045 405 0 g 40 40 404 g 04 g 400 kv 0 kv CS condener Synchronou 0 kv 4 g4 404 4046 04 g6 4 46 g6 Fig.. Nordic tet ytem - ingle line diagram []. i compoed by tranmiion bue with nominal voltage of 400 kv, 0 kv and 0 kv. Additionally, bue are at a ditribution level with a voltage of 0 kv and 0 generator node with a voltage of 5 kv. There are baically three kind of tranformer involved in thi power ytem, the 0 tep-up tranformer convert the generation voltage of 5 kv into the tranmiion voltage that can be 400 kv, 0 kv or 0 kv according to the region level. Eight tranformer convert the voltage among the three different already mentioned tranmiion voltage level. The tep-down tranformer are placed for ditribution dutie and hence their function i to convert the tranmiion voltage level into the ditribution level for the load at 0 kv. All of the tranformer neglect the copper loe and the magnetizing uceptance. The reactive power management i helped by a total of hunt element out of which nine are capacitor and two are reactor. The Equivalent grid ha one hunt reactor, while the North region ha one reactor and one capacitor, the South region doen t have any ort of hunt compenation at all, leaving the Central region which ha to cope with ignificant power tranfer from the North with the eight remaining hunt capacitor. All load are connected to 0 kv bue and are repreented by an exponential model which behavior i decribed a follow: 5 47 g5 4047 Q = Q 0 ( V V 0 ) β () being α = (contant current) and β = (contant impedance). V 0 aume the voltage at initial condition of the bu to which the load i connected. The Nordic tet ytem make ue of hydro and thermal generation. Out of the total of 0 generator, the Equivalent and the North network have hydro generation that i modeled by alient-pole machine. The two bigget generator belong to the Equivalent grid while the North area ha 0 generator. On the other hand, there are even thermal generator modeled by round-rotor machine, two of them belong to the mall South region and the remaining five to the Central one that alo ha a ynchronou capacitor that i modeled a a alient-pole machine. The model for the above lited power ytem element are detailed enough for long-term a well a for hort-term tability tudie. A complete ytem decription can be found in []. III. IMPLEMENTATION OF THE DYNAMIC MODELS All the dynamic model uch a Automatic Voltage Regulator (AVR), Power Sytem Stabilizer (PSS), Over Excitation Limiter (OEL), Load Tap Changer (LTC) and peed governor are decribed in detail in []. Thi ection focue pecially on thoe which implementation can vary from one oftware tool to another. A. Automatic voltage regulator, over excitation limiter and power ytem tabilizer The model depicted in Fig. i ued to repreent the AVR, the PSS and the OEL. The PSS i preent in all of the generator with the exception of the two big generator of the Equivalent grid and the ynchronou capacitor. A firtorder ytem repreent the exciter and the AVR alo include a tranient gain reduction choen to limit the overhoot in terminal voltage following a tep change in voltage reference when the generator operate in open circuit. The PSS i formed by a wahout filter with two cacaded lead which provide damping for ocillation frequencie from 0. Hz to more than Hz. There i more than GW of power tranfer from North to Central area and the maximum power that can be delivered depend motly on the reactive power capabilitie of the Central area. The reactive power limit are defined by the OEL. Thi ytem preent long-term voltage intability given that the maximum power that can be delivered i maller than the one that LTC aim to retore in ome contingency cenario. The OEL are one of the leading force through a longterm voltage collape. In thi particular cae their effect can be eaily oberved when one of the tranmiion line connecting
i fd V ω i lim fd V o Kp Tw T T timer y L T T power ytem tabilizer y 0 y 0 C Min C f 0. 0 G( Ta) Tb tranient gain reduction Fig.. Model of Exciter, AVR, PSS and OEL []. r L v fd 0 0 exciter the North zone with the Central zone i tripped. After uch event, LTC act to retore ditribution voltage and hence load conumption. The generator are forced to increae their reactive power injection in order to keep the voltage at the et-point value. When the field current exceed for a certain time the current limit denoted a i lim fd and equal to 05% of the rated field current i rated fd, the OEL et the field current to i lim fd. The above mentioned time depend on the parameter L, f and r in Fig.. Without an accurate modeling of the OEL the long-term voltage collape to be decribed in Section V will ignificantly differ from the one in []. B. Speed governor and hydro turbine The nominal frequency of the ytem i 50 Hz and i controlled by the peed governor (ee Fig. ) of the hydro generator, the thermal generator of the Central and South area are not involved in frequency control and for the explained reaon in [], contant mechanical torque i aumed for the machine of thermal plant. The model of the peed governor include a imple power meaurement, a PI control and a ervomotor repreented by a firt-order ytem with a time contant of 0. and non-windup limit on z which repreent the gate opening. The peed governor give the gate opening to the hydro turbine model depicted in Fig. 4, which i repreented by a imple lole model with a water time contant T w of. In thi model q repreent the water flow, H the head, P m the mechanical power and T m the mechanical torque. ω σ P P o.0 0.4 PI control Fig.. Model of peed governor []. 0. z 5.0 0. 0 ervomotor z ( q z ) H C. Load tap changer H Tw Fig. 4. Model of hydro turbine []. All ditribution tranformer are equipped with LTC keeping the ditribution voltage in the deadband [0.99 -.0] p.u. The LTC adjut the tranformer ratio in the range [0.88 -.0] over poition (thu from one poition to the next, the ratio varie by 0.0) []. The LTC have intentional delay. When the ditribution voltage leave the above deadband at time t 0, the firt tap change take place at time t 0 τ and the ubequent change at time t 0 τ kτ (k =,,...). The delay i reet to τ after the controlled voltage ha reentered to (or jumped from one ide to the other of) the deadband []. The value of τ and τ are given in [] and differ from one tranformer to another in order to avoid unrealitic tap ynchronization. The implementation of the LTC control wa addreed in thi work with a imple tate machine hown in Fig. 5, which control logic i explained in Table I. D. Saturation The generator main-flux aturation i another iue to be conidered. Small mimatche on the aturation characteritic lead to event hifted in time in a long-term voltage collape when compared with the original ytem. The main reaon i that the OEL will limit the field current at different time if aturation i not conidered or if it differ from the one in []. z00 e e0 z z Fig. 5. State machine for the implementation of the LTC control. z0 q z00 TABLE I LTC STATE MACHINE CONTROL LOGIC. Tranition Condition Action z00 V return to or jump the dead band e=0 z0 e=0 and V i out of dead band for τ e= and tap tep z e= and V i out of dead band for τ e= and tap tep z e= and V i out of dead band for τ e= and tap tep e ω Pm Tm
In thi work, the aturation characteritic i a it i depicted in Fig. 6. V nl [p.u.] Air-gap line. A B C.0.0 i.0 i. i fd [p.u.] Fig. 6. Implemented aturation characteritic. According to [], the aturation characteritic of all generator in the Nordic tet ytem i given by: k = AC AB = m(v nl) n () in which, for V nl =.0 p.u. k =. and hence m = 0., and for V nl =. p.u. k =. and hence n = 6.057. From (), it can be derived that i.0 =. p.u. and i. =.56 p.u. The implemented aturation characteritic i an exponential function with the following input parameter: SG 0 = i.0 i 0 (4) SG = i.. i 0 (5) where i 0 =.0 and hence SG 0 = 0. and SG = 0.. More detail about the aturation model can be found in []. IV. STUDY CASES In order to validate the implemented ytem and compare it againt the reult in [], two different cenario have been conidered. A. Scenario A olid three phae fault i applied to the bu 40 with a ubequent opening of the line 40-4044 without further re-connection. A tated in [], the initial fault i imulated jut to be more realitic, but it i actually the line outage that caue the long-term voltage collape. The long-term behavior would be very imilar without the initial three phae fault. B. Scenario The ame diturbance a in Scenario i applied. In thi cae, a corrective pot diturbance control i implemented. Thi control i a typical Sytem Integrity Protection Scheme (SIPS), which conit in decreaing the voltage et-point of LTC controlling the ditribution voltage by 0.05 p.u. Due to the load characteritic explained in () and (), by decreaing 0.05 p.u. the voltage et-point, the active power conumption i expected to reduce 5%, while the reactive power approximately 0%. The corrective control take place after the lowet tranmiion voltage reache 0.9 p.u. which i at 00 of imulation. Two different variant have been implemented: Scenario a: reducing the et point of the five LTC controlling the ditribution bue,,, 4 and 5. Scenario b: reducing the et point of the LTC controlling the ditribution bue to 5 in addition to the bue 4, 4, 4, 46, 47 and 5. A. Scenario V. RESULTS AND VALIDATION The reult of the Scenario are depicted from Fig. 7 to Fig. 0. For all the figure in thi paper, the original reult taken from [] correpond to olid line, while the PowerFactory implementation one correpond to the dahed line. The chain of event reulting in a voltage collape run a follow: a olid three phae fault i applied to the bu 40. The line 40-4044 i tripped 0. later. The ytem i hort-term table, and it ettle to a new equilibrium in 0. At around 5, the LTC tart acting attempting to retore the ditribution voltage and hence the load conumption. The action of the LTC force the generator to increae their reactive power injection and therefore their field current a it can be een in Fig. 8. Due to the action of the LTC, the OEL of the ix generator in Fig. 8 limit the field current. Some other generator a the one in Fig. 9 are not limited. It can be een in Fig. 0 how the limited generator, a in the cae of g6 and g7, looe their capability to control voltage and their terminal voltage drop until the ytem finally collape. B. Scenario Fig. how the evolution of the voltage magnitude of the tranmiion bu 04 which i the one with the lowet voltage. The action of reducing the voltage et-point of the LTC controlling ditribution voltage can be oberved. The firt variation (Scenario a) of the pot diturbance control coniting in reducing the above mentioned voltage et-point by 0.05 p.u. how that although the control action ucceed in reducing the power conumption, it i till more than what can be provided and the ytem finally collape hortly before 00. On the other hand, the econd variation (Scenario b), how to be an effective emergency control a it i able to prevent the load retoration in a way that the voltage of tranmiion bue i recovered even to a higher value than the one prior the diturbance. Due to the LTC delay, the action of thi pot diturbance control i low but it ucceed retoring the tranmiion voltage before 600 without any under-voltage load hedding. Note that the tranmiion voltage are recovered to value even higher than thoe before the diturbance. In fact, thi emergency control i poible due to the regaining voltage control of the ynchronou generator. A hown in Fig., the generator which were limited in Fig. 8, recover their voltage
Voltage Magnitude [p.u.]..0 0.9 0.8 Bu 04 Bu 04 Bu 40 Bu 406 0.7 0 0 40 60 80 00 0 40 60 80 Voltage Magnitude [p.u.].0 0.9 0.8 No action on LTC Voltage etpoint reduction on 5 LTC Voltage etpoint reduction on LTC 0.7 0 00 00 00 400 500 600 Fig. 7. Voltage magnitude of affected bue Original tet ytem (olid line) PowerFactory reult (dahed line). Fig.. Voltage magnitude at bu 04 Original tet ytem (olid line) PowerFactory reult (dahed line). Field Current [p.u.] 4 0 0 40 60 80 00 0 40 60 80 g06 g07 g g g4 g5 g6 Fig. 8. Field current of even limited generator Original tet ytem (olid line) PowerFactory reult (dahed line). Field Current [p.u.] 4 g06 g g4 g6 g07 g g5 0 00 00 00 400 500 600 Fig.. Generator field current with the implemented pot diturbance emergency control. Field Current [p.u.].0.5.0.5 0 0 40 60 80 00 0 40 60 80 g08 g8 g09 Fig. 9. Field current of three non limited generator Original tet ytem (olid line) PowerFactory reult (dahed line). Voltage Magnitude [p.u.].0 0.9 0.8 0.7 0 0 40 60 80 00 0 40 60 80 Fig. 0. Terminal voltage of two limited generator Original tet ytem (olid line) PowerFactory reult (dahed line). control after ome time avoiding thereby everal over-voltage in the tranmiion ytem. VI. CONCLUSIONS In thi work, the Nordic tet ytem decribed in the technical document Tet Sytem for Voltage Stability Analyi and Security Aement prepared by the Tet Sytem for g6 g7 Voltage Stability and Security Aement Tak Force from the IEEE Power Sytem Dynamic Performance Committee, wa implemented in DIgSILENT PowerFactory. The reult of the implemented ytem are validated againt thoe in the aforementioned report through RMS imulation. Reult how that the long-term voltage collape phenomena can be oberved mainly becaue of the action of LTC and OEL after a ytem diturbance. A corrective pot diturbance emergency control wa implemented and validated againt the original ytem, howing that it can avoid the long-term voltage collape. Furthermore, the implemented ytem can be whether downloaded from the IEEE PES Power Sytem Dynamic Performance Committee webite or requeted directly to the author. VII. OPEN SOURCE The implemented Nordic tet ytem model in DIgSI- LENT PowerFactory can be whether downloaded from the IEEE PES Power Sytem Dynamic Performance Committee webite: http://ewh.ieee.org/oc/pe/pdpc/psdp benchmark ytem.htm or requeted directly to the author. The author requet, that the publication derived from the ue of the downloaded ytem explicitly acknowledge that fact by citing thi paper. ACKNOWLEDGMENT Due to their helpful feedback during the development of thi work [4], [5], the author would like to thank Thierry Van Cutem from the Department of Electrical Engineering and Computer Science at Univerity of Liège - Belgium and Pat-Chair of the Power Sytem Dynamic Performance
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