Real-Tme Control and Protecton of the NEPTUNE Power System Kevn Schneder Chen-Chng Lu* Department of Electrcal Engneerng Unversty of Washngton Seattle, WA 98195 *lu@ee.washngton.edu Tm McGnns Bruce Howe Appled Physcs Lab Unversty of Washngton Harold Krkham Jet Propulson Laboratory Calforna Insttute of Technology Abstract - The NEPTUNE power delvery system faces several challenges n servng the needs of the oceanographc communty. Two major challenges, operatng the system under normal condtons and protectng t aganst faults, requre the development of new approaches unfamlar to power engneers. In partcular, the power management system must cope wth several modes of potental system nstablty, and the protecton system must operate n a delberately weak system. Furthermore, t s lkely that communcatons wll be dsrupted n the event of a fault. The approach taken to address these challenges s descrbed. I. Introducton In the past, power lmtatons have restrcted long term oceanographc studes to usng only low power nstrumentaton. NEPTUNE seeks to relax the power constrant by extendng the capabltes of the conventonal terrestral power delvery system grd nto the Pacfc Ocean [1, 2]. Terrestral power systems are based on nterconnected ac networks wth parallel loads, whle underwater telecommuncatons are dc pont to pont seres systems. The proposed NEPTUNE power system s dfferent from both. It s a hghly nterconnected dc system wth parallel loads. It wll consst of a 3000 km cabled sub sea network wth two shore landngs that wll supply power at approxmately forty-sx locatons, see Fg. 1. Each of these forty-sx nodes wll provde a pont of nterconnecton for scentfc equpment, supplyng both power and communcatons. In order to maxmze the delverable power, the system wll operate at -10 kv wth respect to the ocean. The voltage suppled to the scence load wll be 400V and 48V va power converters. Fg. 1. The proposed NEPTUNE observatory n the northeast Pacfc Power s suppled to the system from two planned shore statons, one n Oregon and the other n Brtsh Columba. The system wll use a sngle conductor telecommuncatons cable, referred to as the backbone cable that requres a sea water return system. 1
B ackbone B reaker 10 kv B ackbone C able 10 kv B ackbone C able 10 kv S upply B us Converter 1 Converter 2 S tart-up C rc u ts 400V S upply B us In te rn a l Loads ExternalLoads E nergy S torage (400V B us) E nergy S torage (P rotecton) P rotecton Fg. 2. Internal node arrangement In order to supply power to the entre system, t s planned to energze nodes sequentally from shore. Once power s appled to the frst off-shore node through the backbone cable, the man power converter start-up crcuts are suppled through dodes (Fg. 2). The purpose of these dodes s to allow power to be suppled to the start-up crcuts from ether end of the backbone cable. Ths s necessary snce the networked topology of the NEPTUNE system makes t mpossble to specfy n advance whch sde wll be energzed frst. After approxmately 10 seconds the start-up crcuts causes one of the man power converters n the node to begn operaton. As soon as the man power converter s operatng, the communcatons system begns ts own start-up perod. The communcatons start-up has a duraton of between one and two mnutes at each node. Wth the establshment of communcatons, the backbone breaker s closed and the next node can be energzed. Once power and communcatons have been suppled to a node, external load can be suppled. The backbone breaker s actually a complex system of several swtches. When closng, a pre-nserton resstor wll be used to lmt the current through the breaker. Ths serves two purposes. Frst, t lmts the volt-drop on the precedng cable that would be caused due to chargng the capactance of the next secton. Second, t allows for the protecton system to detect a cable fault before full power s appled. The full closng sequence wll occupy only a few mllseconds. By repeatng ths sequence, the NEPTUNE system can be completely nterconnected. Snce NEPTUNE s a network, t wll be able to operate wth multple nodes and/or cables out of servce. Ths feature of the power system wll allow for relable delvery of power. It should be noted that durng a normal start-up sequence, the backbone crcut breaker s not closed untl communcatons s establshed wth the shore-based power management system. Snce the proposed communcaton system takes a mnute or so to execute a cold start, the process of startng NEPTUNE may occupy several tens of mnutes. Should the communcaton system at a partcular node fal, the power system controller n the node wll (after some tme) close the breaker wthout beng so nstructed from shore. It wll enter what we are callng a safe mode. 2
Some of the protecton system s operatng, and as much as possble of the power system s energzed. Durng the tme that nodes are beng energzed, a power management system ensures that the system s not beng placed n an unstable condton. Stablty must also be montored durng routne operaton. Our approach s descrbed next. II. Power Management Power flow, or load flow, calculatons are the prmary tool for calculaton of the steady-state operatng condtons of a power system. The power flow problem s non-lnear, whch ncreases the complexty of calculaton. Several well known numercal methods are avalable for soluton of the problem. The method adopted n ths paper s the Newton-Raphson (N-R) method that has been wdely used for conventonal alternatng current power systems. Power flow for terrestral alternatng current power systems has been extensvely studed and s well understood. Power flow for drect current systems s an area where relatvely lttle work has been done, for ths reason t wll be addressed n ths paper. It should be noted that relatve to the conventonal power flow algorthms, the NEPTUNE power flow has the followng advantages: In the ntal teraton, V N where N s assumed to be 1 for the ntal teraton, all buses are assumed to have 1 per unt voltage, whch corresponds to 10,000 V DC. Ths s used as the bass to calculate the new voltages, V N+1, at each of the nodes n the system. These new values are then desgnated as V N and the process repeats. Convergence s acheved when the msmatch f(v N ) s reduced to wthn a prespecfed tolerance. To ensure accuracy of the calculatons, the stoppng pont for the teratve scheme was set by the sum of the (absolute value of) dfferences of voltages at all buses, nstead of just one reference bus, from one teraton to the next. Usng the dfference of voltages at all buses tends to ncrease the number of teratons requred for convergence, ths however s not a major concern snce the NEPTUNE system s small and the computaton tmes short. When the load on the power system s ncreased, there comes a pont at whch the load flow problem ceases to have a soluton, the calculaton wll not converge. Ths s easly llustrated by consderng a stuaton n whch there s a constant voltage source supplyng a load through a transmsson lne (Fg. 3). Fgure 3 shows a condton where a constant voltage source delvers power to a constant power load. As the value of the constant power load s set at hgher levels, more current must flow across the lne, lowerng the receved voltage. 1) As t s a dc system, there are no reactve power consderatons. 2) As t s a dc system, the nductance of all lnes can be consdered zero at stead y state. 3) DC systems have no voltage angles to calculate. Constant Voltage DC Source (V S ) V S + - Lne Resstance (R Lne ) V R Constant Power Load (P Load ) at Voltage V R Together these factors ndcate that all that needs to be solved for s the voltage at each bus, based on real power njectons nto the system. The node voltage V at teraton N+1 s gven by equaton: V N+1 = V N - N -1 N ( J(V ) ) f(v ) (3.1) Fg. 3. Transmsson lne model The voltage at the recevng end of the lne, for a gven source voltage and load power, s gven by: where: = Voltage at begnnng of the teraton. = Voltage at end of the teraton. f(v N ) = Power flow equatons. J(V -1 ) = Jacoban of the power flow equatons. V N V N+1 In the above equaton, the msmatch at bus s gven by: f (V where: N ) = P - P L = P - V L n Y 1 k V k P L =The njecton of power nto bus. n=number of nodes V R where: 2 =.5 (V S ± V S - 4P L R L ) (3.3) V R = Voltage at the recevng end V S = Voltage at the sendng end P Load = Power of the load R Lne = Resstance of the lne (3.2) In Fg. 4, the voltage s plotted as a functon of the load power. As wth the power flow calculatons, the P-V curve s well known for ac system but s seldom appled to dc systems. 3
V R Stable Operatng Pont Regon of Maxmum Power Transfer and the fault has been cleared, a system restoraton procedure must be followed. Ths s also descrbed below. Status Data and Analog Measurements Voltage, Current, Power Lmt Checkng Unstable Operatng Pont Fg. 4. P-V curve P LOAD Securty Assesment Emergengy Control System Restoraton All Operatng Constrants Satsfed Operatng Constrant(s) Volated Loss of Elecrtcal Load(s) Snce Equaton 3.3 s quadratc, there s a possblty of zero, one, or two solutons for a gven load power. Only the stable operatng pont s physcally realzable. The pont of maxmum power transfer marks the maxmal power beyond whch power flow soluton ceases to exst. Ths corresponds to the pont n ac systems at whch voltage collapse occurs [6]. Extensve smulatons have shown that voltage collapse s not a major concern, n the ntal desgn, for the NEPTUNE power system because the shore staton converters are ncapable of delverng suffcent power. (At present t s proposed that the shore statons be rated at 100 kw each.) In the present desgn, the total load capablty of the sub-sea converters s around 920 kw, assumng operaton of both converters n a node, far larger than the capablty of the delvery system. Because the rated shore staton output power may be ncreased at some future tme, voltage collapse s beng studed now to determne what the non-constraned system lmts are. Voltage collapse may also become an mportant ssue when the power system s operated n other than fully networked topologes. For example, when the system s operatng wthout the Oregon shore staton, the maxmum power capablty of the system (lmted by voltage collapse consderatons) s 1.93 kw at each node. Ths fgure s wthn the capablty of the Vctora shore staton, the lmtng factor s the voltage collapse lmt. Montorng the power system to ensure that the operatng constrants are met s the task of the proposed Power Montorng and Control System (PMACS). The software functons of PMACS are shown n Fg. 5. Through the communcaton systems, status data (for example, breaker status) and analog measurements (voltage, current, and power measurements) are acqured and archved by PMACS. If all operatng constrants (voltage, current, and power lmts) are satsfed, the system s n a Normal state. In case any of the operatng constrants are volated, the system s n an Emergency state. The actons taken by PMACS n these states are dscussed below. If servce to loads has been nterrupted due to a fault Fg. 5. Power Montorng And Control System (PMACS) A. Montorng and Emergency Control PMACS has Supervsory Control And Data Acquston (SCADA) capabltes. (The SCADA system s a common remote montorng and control system for electrc power systems.) Remote control capabltes are needed so that the shore staton can ntate necessary swtchng actons such as remedal actons to allevate an overload or abnormal voltage condton. 11000V DC Voltage 5100V Fg. 6. Allowable voltage band 1 2 3 4 5 6 7 Node Number Because the resstance of the cable proposed for NEPTUNE s large, voltage drops from the shore staton to the remote locatons wll be sgnfcant. Whle each dc-dc converter has an nternal control loop to regulate ts load voltage, t s desgned to shut-down f the nput voltage drops below 5,100 V. (Note that ths acton would help preclude voltage nstablty, as system voltages around 0.5 p.u. ndcate the onset of such nstablty.) PMACS wll montor the node voltages and determne f t s necessary to adjust the source voltages or the loads so that the voltage profle along the cable system remans wthn an acceptable range at all tmes. An unacceptable voltage profle s 4
llustrated n Fg. 6 whch shows the system voltages at nodes 1 thru 7, the seven southern most nodes n Fg. 1 wth node one beng furthest south, when the average load on each node n the system s 4.9 kw. In order to obtan the results n Fg. 6 converter dynamcs were not consdered,.e., the load was not reduced to zero when the converter nput voltage dropped below 5100V. The shore staton wll acqure voltage and current measurements from all nodes of the backbone once every second. The PMACS software wll perform lmt checkng to detect any voltage volatons or over-current problems. If any abnormal condton s detected, the emergency control actons are determned by the PMACS at the shore staton. The emergency control actons can nclude adjustment of the shore staton voltages or load sheddng. The algorthm to calculate the emergency control actons has yet to be developed. Whle load sheddng would only be used as a last resort, the NEPTUNE power system wll categorze loads. At present four categores are antcpated: 1. Essental loads are crucal to safe system operaton. These loads are generally nternal to the node, for example, the communcaton system. 2. Hgh-Prorty loads warrant extra effort to keep them energzed. Ths s lkely a category that ncludes temporary desgnatons, as events n the observatory change. General loads wll be temporarly desgnated Hgh-Prorty f the scentfc nterest warrants t. 3. General loads wll receve no partcular effort to keep energzed, nor wll they be early on the lst of loads that can be shed. Most scence loads are expected to be n ths category. 4. Deferrable loads are canddates for dsconnecton as the system approaches peak power. Lghtng and battery-chargng systems are lkely to be n ths category. B. Securty Assessment PMACS wll have a securty assessment module. Securty s a measure of the power system s ablty to wthstand a contngency, such as a short crcut or the loss of a shore staton. Fgure 7 shows how the PMACS securty assessment module categorzes stuatons. Normal condtons can be dvded nto two categores: secure and nsecure. In both of these cases the system s operatng wth no operatng lmts volated and all load beng suppled. The dfference between the two categores s how they compare wth respect to a pre-selected lst of contngences. (The contngency lst has not yet been selected.) Emergency Fg. 7. Securty assessment Normal Condtons Secure Insecure Emergency Control Acton Restoratve Control Acton Restoratve To be n a secure operatng state the system must meet all system operatng requrements at the present tme, as well as meetng all requrements f a fault from the contngency lst occurs. If the system meets all requrements at the present tme but would volate a lmt should a fault from the contngency lst occur, the system s nsecure. In both cases, secure and nsecure, all system loads are suppled. Durng an emergency condton operatng constrants are not met. The volaton of operatng constrants could be n the form of a low voltage or a hgh current. From the emergency state the system can ether move back to a normal condton, where all constrants are satsfed, or progress to a restoratve condton where load has been lost. At ths pont the restoraton component of PMACS s actvated. C. Restoraton Even though the NEPTUNE power system wll be equpped wth a fast-actng protecton system, shut-down of much of the network s scence load may be unavodable n the event of a fault. Ths s because the shore statons voltage wll automatcally (and rapdly) drop n order to acheve current lmtng. Consequently, many of the dc-dc converters wll stop operatng. Snce t s not planned to have a large amount of energy storage as part of the sub-sea system, wdespread outages of scence loads wll unavodably result. Therefore, after the system has fnshed respondng to a fault (.e., the protecton system has operated to solate the fault), some parts of the system that are not faulted wll have to be brought back on-lne. In the worst case, the entre system wll requre re-start. A better restoraton system would mnmze the tme to recover from a fault. To accomplsh ths, some means of re-startng the entre system at the same tme would be needed. A re-start would then be a fundamentally dfferent process from a normal start-up. Such a restoraton system s beng desgned. 5
III. Protecton A utlty power system typcally delvers power to ts customers wth a hgh level of relablty. Ths level of relablty has not been acheved through a lack of power system faults. Instead, t s acheved through redundancy of parts and an atttude of f t can go wrong, t wll. Ths atttude has led to the use of N-0, N-1, and N-2 relablty crtera. These crtera refer to the number of power system components that are lost before a loss of load s experenced. N-1 and N-2 are generally accepted for the nterconnected transmsson systems. Dstrbuton systems, due to ther radal nature, tend to be relable only for N-0,.e. no component falures! We would lke to buld NEPTUNE to meet N-1 relablty crtera, that s to be able to serve load even f some part of t s out of servce. The goal of the proposed protecton system s to dsconnect a faulted cable secton or a component, wthout affectng the remander of the system. From the begnnng, t has been clear that the NEPTUNE power system wll be dfferent from a conventonal power system n many ways. The major problem from the aspect of protecton has been the ssue of a weak system. When a fault occurs on the NEPTUNE power system, the voltage rapdly collapses across a large porton of the system. Node power converters cease to functon, wth the result that there s no longer a source of power for the protecton relays at those nodes. In a terrestral system, each major substaton contans banks of lead acd batteres that act as a source of stored energy for the protecton relays. In NEPTUNE, t s not feasble to nclude large banks of lead acd batteres. Whle alternatves are beng evaluated, t s presently thought that t wll not be possble to accommodate suffcent energy storage to allow for many of the conventonal protecton relayng schemes. The three areas n the NEPTUNE system that requre protecton are: the 10 kv backbone system, the nternal node components the 400/48V low voltage dstrbuton system. These areas wll be dscussed separately. A. Backbone Protecton In order to protect the backbone cable aganst shunt faults, a redundant relayng approach s proposed. The two redundant methods are dfferental current and dstance relayng. Dfferental current relayng works on the prncple of that the total current enterng one end of a cable should equal the total current leavng from the other end. If there s no load connected between the two ends, ths condton s volated only when there s a fault. See Fg. 8. Snce there s a latency assocated wth travel tme of nformaton from one node to the next, all current measurements are tme stamped so that the correct data are beng compared. One of the great advantages of the dfferental current protecton s that t provdes dscrmnaton for hgh mpedance faults. Data Tranfer Path Protected Backbone Cable Current 1 Current 2 Node 1 Node 2 Fault Current Fg. 8. Dfferental current protecton Current Sensors Dstance relayng works on the prncple of measurng the resstance of the cable. At each pont n a node where a backbone cable penetrates the pressure case, voltage and current measurements are made (Fg. 9), so that the value of resstance can be determned. In a normal operatng condton, ths value s greater than the resstance of the cable secton. Durng a fault condton, assumng a zero resstance fault, the resstance value seen from the source drops, and can be used to estmate the dstance to the fault locaton. For most utlty applcatons the reach of dstance relays s set to 80% of the dstance to the next breaker and relayng pont. A value less than 100% s used so that a fault close to the far end of the secton, but on the other sde of the next breaker, wll not cause 2 breakers to trp. The accuracy n estmatng the dstance wll be lmted to the accuracy of the current and voltage sensors and the fact that the sensors are measurng transent waveforms P rm ary Z one R each P rotected B ack B one C able C urrent and V oltage S ensors Node 1 Node 2 Fg. 7. Dstance protecton, set to around 80% B. Node Protecton Protecton aganst faults wthn a node s easer than t s for the backbone cable. The reason for ths s the physcal proxmty of components. Dfferental protecton s feasble usng no more complex a communcaton system than a meter or so of wre. For example, even though the typcal NEPTUNE node has 3 wres assocated wth ts backbone crcut (ncomng 10 kv, outgong 10 kv and a ground), the current n these 3 can be added n a devce called a dc current comparator, and even a small dscrepancy can be detected. One of the tasks of the NEPTUNE Power Group wll be to develop a low-cost comparator for sub-sea use. Whle a comparator wll detect a ground leakage wthn the node, detectng a fault nsde a converter s more 6
dffcult. Converson of voltages from the varyng 10 kv backbone level to the constant 400V and 48V scence voltages amounts to a transformer acton, where the transformer rato s not fxed. Thus, nstead of current comparson, power comparson wll be used to relay the converters. The method of power comparson requres both current and voltage measurements, and requres that the power lost n the converter as thermal losses be taken nto account. Whle converter effcency vares from approxmately 92% to 94% dependng on loadng, we feel that the problem s solvable. IV. Concludng Remarks Ths paper represents the contnuaton of work that has been n progress for over a year. A major contrbuton made by ths paper s the outlne gven for both the power management and protecton components of the NEPTUNE power system. Analyss has shown what the lmts of voltage stablty of NEPTUNE are and PMACS has been desgned to work wthn these lmts. In addton to the normal operaton, provsons have been made for PMACS to control the system durng emergency condtons and to restore lost sectons of the system after faults have occurred. A protecton system has been proposed that wll operate quckly n order to solate faults n the mnmum possble tme n order to mnmze unnecessary loss of load. There s stll a great deal of work that remans to be done before the deas presented here can be mplemented n an operatng system. Examples are: What wll be the voltage lmts for emergency control? What faults wll be on the securty assessment lst for contngences? What wll be the exact protecton relay settngs? What amount of energy storage s requred n a node to ensure contnuous operaton of the protecton system? These questons, and several others, wll be the center of the research that wll lead to the fnal desgn of a power system that s capable of supportng the NEPTUNE system. Acknowledgment Ths research s sponsored by Natonal Scence Foundaton through the grant ttled, The NEPTUNE Plate-Scale Observatory: The Power System, OCE-0116750. References [1] B. Howe, H. Krkham, and V. Vorperan, Power System Consderatons for Undersea Observatores, IEEE J. Oceans Eng., Vol. 27, No. 2, Aprl 2002, pp. 267-274. [2] B. Howe, H. Krkham, V. Vorperan, and P. Bowerman, The Desgn of the NEPTUNE Power System, Proc. Oceans, 2001, MTS/IEEE Conference and Exhbton, Vol. 3, 2001, pp. 1374 1380. [3] J. Delaney, G. R. Heath, A. Chave, H. Krkham, B. Howe, W. Wlcock, P. Beauchamp, and A. Maffe, NEPTUNE Real-Tme, Long-Term Ocean and Earth Studes at the Scale of a Techtonc Plate, Proc. Oceans 2001, MTS/IEEE Conference and Exhbton, Vol. 3, 2001, pp. 1366-1373. [4] A. R. Maffe, A. D. Chave, G. Masson, S. N. Whte, J. Baley, S. Lerner, A. Bradley, D. Yoerger, H. Frazer, and R. Buddenberg, NEPTUNE Ggabt Ethernet Submarne Cable System, Proc. Oceans 2001, MTS/IEEE Conference and Exhbton, Vol. 3, 2001, pp. 1303-1310. [5] C. C. Lu, S. J. Lee, and S. S. Venkata, An Expert System Operatonal Ad for Restoraton and Loss Reducton of Dstrbuton Systems, IEEE Trans. Power Systems, May 1988, pp. 619-626. [6] K. T. Vu, C. C. Lu, C. W. Taylor, and K. M. Jmma, Voltage Instablty: Mechansms and Control Strateges, Proceedngs of the IEEE, Volume 83, Issue 11, Nov. 1995, pp. 1442 1455. 7