"Applications and Advantages for Protection schemes using IEC 61850 Standard"by Brent Duncan & Howard Self

Siemens PT&D

(Abstract) Modern substations which are being built today use sophisticatedmicroprocessor based relays. These relays have drastically reduced the cost ofsubstations by combining multiple protection elements into one device.Additional enhanced functionality has also eliminated the need for separatedevices such as measurement transducers, Sequence of Event recorders, largepoint count RTU's (Remote Terminal Units) and, Transient Fault Recorders. Untilnow complicated control schemes, such as, an auto transfer scheme or a throwover scheme for Main-tie-Main Switchgear, or reverse interlocking bus protection,still required copper wires, auxiliary and lockout relays to achieve high speedreliable functionality. Implementation of the new IEC 61850 (InternationalElectrotechnical Commission) standard will allow this to change.

(Introduction) A large number of different protocols for substation communication arepresently available. Until now, they only existed to provide data to control centers anddata repositories. The new communication standard IEC61 850 which was developedand expanded from the UCA 2.0 protocol, allows the communication capabilities of all ofthe IED's (Intelligent Electronic Devices) in a substation, or even across the entire powernetwork to directly communicate with each other. A relay that can now send and receive"GOOSE" (Generic Object Oriented Substation Event) messages for peer to peer relaycommunications, send fault records automatically, and communicate to IEC 61850station master, over a high speed LAN, (Local Area Network) will reduce cost byeliminating conventional wiring. These advancements in high speed data transmissionwill be recognized immediately by the customer. This paper will show applications thatpreviously relied on copper connections, auxiliary relays and lockout contacts, and howthese can be implemented and monitored over a high speed Ethernet LAN. Using adual fiber optic high speed LAN is much more cost effective and as efficient and reliableas copper. The programming software to achieve the configuration and the softwaretools to monitor and evaluate these functions is of vital importance. This must be intuitivesoftware and allow the user to define which information is passed between relays aslogical nodes. Troubleshooting the communication will be substantially easier withsoftware that can identify reception as well as transmission of data. Specifically, we willinvestigate the reverse interlocking bus protection application and review requirementsfor an auto transfer scheme for Main-tie-Main Switchgear.

.. ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.._.......................

_ l~~yse ( cotrl1

StandardizedX

.. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.._.......................

languagefo1autreod

describing ) IEC 61850inComtrade~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~............................

Standard (~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~..........................communication Time~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...........................C Ethernet~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~,with SNTP~~~~~~.........................................

Figure1.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~......HHHHHHHH

Software tools required

The programming software requirements are such that it must allow the setting andconfiguration of all aspects of the device. This is defined as an ICD file (IED CapabilityDescription). Further it is required to define the entire system configuration as a SCD file.(Substation Configuration Description)This file can be archived, retrieved and engineering companies can standardize ondesign. The advantages of having user friendly MHI software as a support tool for settingdigital multifunction relays and how the end user can benefit from the softwareenhancements are a reduction in the time required to configure the device and system.The value of HMI software packages are becoming more and more important today tosave time and money during Design, Testing, Commissioning and Maintenance.This is mainly due to the increased complexity of digital protection relays or I"ED's" asthey have integrated Protection, Metering, Control/SCADA and Logic/PLC Programminginto one device. With the evolution of the Induction Disk relay into a microprocessorbased IED, the algorithms or software programs for over current protection simplyemulate the physical properties of these earlier induction disk devices. One of the mainadvantages is that due to multitasking capability many programs can run at once. Thedevice can now be multifunctional, and the hardware requirements are greatly reduced.The disadvantage is that the number of settings also increases. In earlier generationseach setting had a specific address therefore this resulted in an incredibly large numberof time consuming settings. With the innovation of the new software the protectionengineer needs to be able to configure a digital relay easily and in as little time aspossible. The protection engineer has many other critical tasks such as: to providesupport to his customers, do expert analysis quickly and efficiently to ensure minimaldown time after a fault event and provide professional reports. This is quite a task, butwith today's software complete "Electronic Management" of relays is possible. One ofthe newest innovations is that configuration using a MATRIX not Menus is used. In thisway all of the options for Binary Inputs, Binary Outputs, LED's, even as detailed tomapping standard protocols is visualized. The MATRIX offers a complete view of relayconfiguration, with Filters to minimize displayed information.

__________ ___ __. . _

Filters

Hardwrarr.EE ll

Figur 21k. 6

. F IIIIII~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~..................................._:_r IIIIII-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...................................X L i.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~..................

*1MQElgl^ima@WX6!4j i~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~..................Figure2.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~..................................

In today's multifunction relays the user has hundreds of variables to contend with. Thechallenge is therefore, how to manage these settings in such a manner that takes intoaccount the fact the human element (e.g., a person has to interact with the settings).The most basic method is a carry over from a bygone era - program the relay with linesof code (similar to how we all learned to program with Fortran77). A more recentdevelopment is a product of the Windows convention, have the user adhere to a menustructure. The pitfall of both of these methods is that this is not how the human brainworks, the human brain is apt to change focus in a split second and want to seesomething else. The 'lines of code' method is counter intuitive (unless the user favorsprogramming languages), and the menu structure method locks the programmer deepinto the realm of a specific section of the relay configuration. Neither method providesan overview of how the relay is configured nor does either method allow the user toquickly shift thought processes.

A result of new programming software is that it has evolved past these cumbersomemethods and the result is a Matrix. The Matrix is a tool that allows the user to shift fromlooking at command values to just binary 1/0 with a click of the mouse, and more: fromthe matrix the user configures the 1/0, creates (custom) user variables, programs theannunciation LED's, selects of the 1/0 designated for the built in PLC, with a double clicklaunch directly into ANSI protection function Setting screen, and much more. The Matrixcan be thought of as a spreadsheet with links to all aspects of the relay.

A product evolves to offer a mix of functions to meet market requirements. Some designsoftware glitches are inevitable so firmware revisions become a requirement. Allfirmware upgrades & down grades need to be available from the Internet. These aresimple functional upgrades performed by the user.The support of a critical device requires facilities built into the device to Export Files andsupport E-mail compressed files for immediate support and analyses. The function ofImporting Files must allow you to Compare Setting Files with Relay settings.The program needs to be integrated into the automation software of the PLC's.The communications cards in the relays have their own processors. This means that therelays can support multiple communication Protocols, they are interchangeable andupgradeable. A customer who has not yet decided on the SCADA system can stillproceed with the relays and add the specific communication card at a later datedeferring the cost and protection the hardware investment.The evolution of this configuration extension software to the basic setting software camefrom a DOS based menu driven software to a Windows based menu driven software,with Inclusion of all previously released firmware versions in one software package forCommunication to all RelaysIt is now a Windows based MATRIX driven software which communicates to earlierversion devices. By use of the CFC (which stands for Continuous Function Charts), andis based on the IEC 1 1 31 standard logic can be implemented. The CFC is used inaddition to MATRIX for additional logic and for Interlock capabilitySystem Interrogation and Graphical Representation Analysis tools are used for forensicsfault recording analyses. A convenient feature is that it can import & export Comtradefiles. Various manufactures waveforms can be analyzed in different visualizations. It canalso synchronize multiple files for different relays, and allow superimposition ofwaveforms on top of each other. Multiple Views of the Vector Diagram, Circle Diagram,Harmonics and other are available for visualization and reporting. Programs handle notonly curves, but also circle diagrams, vector diagrams and bar charts. Using themeasured values which are registered in the fault record, calculation of further values,such as positive-sequence impedances, R.M.S. values, symmetrical components, andvectors can be visualized. Accessories sucIps, two measuring cursors enable thecapability to configure the fault record quickly and comfortably. Other fault records, for

instance, from the remote end of a line can be added to the default one. Asynchronization function makes it possible to synchronize the fault records to a commontime basis. There is only one restriction: the fault records have to be provided with theworldwide standardized COMTRADE format.

Software such as this allows for today's E-Support Tools to be used to remotely supportthe product. We now have the capability to Import /Export single device setting files andalso Import/Export multiple device setting files. We can use the software to providecomparison of setting files copy and paste device files. Device file upgrades andfirmware upgrade and documentation are available online and free of charge.

Reverse interlocking

The term reverse interlocking is referenced by several terms in the industry. Zonesequence control Interlocking or Poor man's bus protection are two of these.Through the use of IEC 61850 parallel hard-wiring between devices can be replaced bysimple Ethernet cabling for implementing this application. GOOSE (Generic ObjectOriented Substation Events) messages are exchanged with the same degree of securityas hard wiring. The scheme is configured with software on a Personal Computer (PC)Reverse interlocking allows for cost effective busbar protection on high and mediumvoltage switchgear. Single ended in feed will allow the explanation of theimplementation, but other topologies can facilitate this application as well. Referring tothe fig 1 .x we see that over current relay coordination is the basis for the application.

Principle of reverse interlock(ANSI# - numbers)

Main Incoming

OC-Feed4 >Block #50-2

D 50-lOOm lokngPower flow Sga

~~~~~1 1#50-I pick d up 50-1 pick up #50-1 picke up

-Fl -D F3I~~~~I

Figure 3. 66

For an external short circuit on a feeder, the instantaneous over current protectionANSI#50 on the main is blocked by the feedback of a binary signal that the feeder relayhas picked up. In the traditional scheme this is done with copper wire. The IEC 61850allows this pickup to be defined as a logical node (PTOC) following the standard. A timedelay in the range of 50 to 1 0Oms is set on the incoming main to ensure the feeder willblock in sufficient time. The pick up of the ANSI#50 high set stage must be setsignificantly higher than the over current pickup in the feeders to ensure propercoordination. The short circuit is cleared by the over current protection in the feeder bayin which the fault occurred. When the pick up of the normal stage resets the blockingcondition also resets as the fault is no longer present.

Short circuit on the bus bar.

The instantaneous high set stage element ANSI#50HS of the main incoming is set sothat it will detect faults on the bus bar. None of the normal pickup stage will function on afault on the bus. When the time delay has expired a trip command is issued to the mainincoming breaker and the bus fault is cleared. To implement this scheme with hardwiring we must apply a voltage to the binary input. This is referred to as wetting thecontact. The contacts signaling the ANSI #50 pick up in the feeder bays PTOC.str,PTOC.str2and PTOCN.str are logically combined with an OR function to the ANSI#50HSblocking binary input of the main infeed. The closing of one or more contacts results inthe blocking of the ANSI#50HS via the binary input. If the over current protection devicein a feeder is temporarily out of service and a short circuit occurs on the feeder duringthis time, no blocking signal can be generated, and the bus bar would be tripped. Anyinterruption of the connection between the feeder and the main incoming will have thesame undesired result. These conditions can now be detected and handled by the IEC61850 based interlocking. This is superior to the hard wiring approach. Below in thefigure we see proof with an instance from a number of repeated tests showing that the61850 communicated output trip is faster than the copper. Although this is a simplenetwork with limited devices, this beat the wire trip every time. The signal Ext_TRP_In isthe signal to initiate trip. So we can see that the Remote_Tr_A from the IEC 61850 wasfaster than the copper connection.

Fille Edit Insert VieW OpbbMns Windbiw Help

L~ El K + hI.fl Dj it <LIet Lotq1;in - ; <uigi ~~~ie

. _h ft§ Meosueng Sigridi ihl~ardelnaow§ RM& ph-§ Nel F_b-ft FbId& TimeTe&_7SJ60_VUV6Ufll3Filename. C-S1EMENDIGSMDIPRJFEB20TH\P7 DI\G)ASTR000t00J01\SAMPLES FAULFF

oe__...................................F..en..e. 2: 1SEh18811EEg\21P:O1FE2gXeMl7.7EiSOO3ODNonetFL551iLll

~ 14.6

|I-D1D 0Hof 01 1: 020 030 0140 o150 MO ts

-0.100 .0 |0.10 0.20 OM0 0.40 160 HO6 U

FlagLLos

Arc fult

test ninl ____________

firitTIrI:lE t TFFR hi'

Ji lU011 02 ['4 [15?70nUs

ForHelpJ pres 5 Fl 67 ecriary 1h: 60CO Hz.Pri: 13000 M/100C }Sec 120ii'C1f5 IH-

Using the IEC 61850 Station Bus for implementation of this application uses GOOSEmessaging. This allows for information to be exchanged between devices over theEthernet network. The GOOSE transmission characteristics must be set with theconfiguration tool. For this we will assume that the first repletion time is equal to 5ms,and the last repetition time equal to one sec. Any ANSI#50 pick up on the feeders resultsin the blocking as well as the associated Quality attribute and time stamp and will betransmitted as a GOOSE message on the Station bus.See fig 3b.GOOSE

Transmitter Service:IEC61850 GOOSE Receiver of GOOSE -messages

PTOC1 Status supervision ServiceJPTC1

* IEC6tvalid / unvalid IC 8 r

PTOC2 ~ POC tq POLM&MID'./PTOC2 2btocked

....I T .St_.t_._

I .J../PTOCN.Stx.general.J.IPTOCNStr.q

PTOCN

Figure 3(b): Blocking - method realized with the GOOSE - service of lEC 61850

These GOOSE messages are all received and interpreted by the device in the mainincoming bay. Here the pickup signal and the associated quality attribute "cvalid"9aretransferred to the internal logic function (PLC) of the device. Using this means ofprogrammed logic the described conventional hard wired system can be emulated asfollows: Any pick up (Str of PTOC1 -PTOCN results in the blocking of the main incomingPTOCi. The blocking at the in feed main is made available as an IEG variable PTOCi.Mode-stval=2 and can be reported to an IEG 61850 client, functioning as a SubstationController. Should one of the feeder over current devices fail, the status of theassociated pick up signal is set to invalid should the time out time which is 2 x the lasttransmission time expire. This change in status can also result in blocking of the main ora failure alarm being annunciated. If the communication link between devices fails, thiscondition will also set the pick up to be set to invalid. This logic emulates the response ofthe hard wired system. An additional advantage is the supervision of the data link due tothe status information. Also not to be overlooked is the fact that parallel wiring is notnecessary. The GOOSE messaging must be correctly transmitted with in 10 ins. TheIEG 61850 has made special provision for this prioritized data transfers.

Network Topology

prvd time/synchrnizatineain th ewr ih1m cuay.Hrwr itr loomesgst be only rcIve at th drseddvcsOpticalconeton makes th oprto reial an lupshetmnsoconentona cope concin.SeFg .B en fte igtplg n

comprehensive monitoring functions the failures of components can be detected within afew milliseconds.

Central unit with. time synchronization, and network supervision

Optical fibre

Operat Ptch Ethernet patch cablewith Multimode - fiberDIGSI 62,5/125 pm or 50/125pm

PTOC1 PTOC2 PTOCN PTOCI

Figure 4: Physical arrangement of the Ethernet network

Main Tie Main Auto transfer Scheme

Many industry and utility distribution substations use a Main-Tie-Main bus arrangement,where two load buses, supplied by different sources, are linked by a normally open tiebreaker. Upon the loss of one source, closing the tie breaker allows the other source tocarry the entire load. It is also common practice to implement an Automatic Transferscheme, where if one source is interrupted, a control scheme will move all load to theother source by automatically closing the tie breaker. These are sometimes called throwover schemes.Traditional automatic transfer schemes link protective relays, selector switches, breakercontrol switches, auxiliary relays, and timers through a complicated contact logicarrangement. These schemes are time-consuming (and therefore expensive) to design,wire, and maintain. The schemes are also difficult to test, maintain and verify, as manydiscrete elements are involved.

| ~ ~ ~~T,ie Ellker |

Fig 5 cnre Lirme Diaaratrn - Traditiornal EVlairi-Ti;l~%/airil tto

This application describes the use of Multifunction Over current Relays to perform atypical automatic transfer scheme. By using the protective elements and programmablelogic capabilities of the IED, all auxiliary devices, including switches, timers, and relays,can be eliminated. By performing all the logic in software, design time is reduced,replication of schemes is simplified, and modifications are quickly performed, as thephysical system remains constant. The use of the IED permits complete annunciation ofthe transfer scheme operation, simple testing, and graphical test verification reports, aswell as a dynamic display of the system status. The main advantage is the reduction incopper as the required signals are now available to the relays as peer to peerinformation (GOOSE) messages. The typical main-tie-main automatic transfer scheme inuse combines protective relays, breaker control switches, auxiliary relays, and timers ina contact logic design. This requires a separate control logic design for each incomingmain breaker and the tie breaker. When the scheme is in automatic mode, loss of sourceat one incoming main initiates automatic transfer. The scheme opens the main breaker,and then closes the tie breaker. The scheme must differentiate between a system faultand a loss of source. Automatic transfer is normally blocked if the other incoming mainalso detects a loss of source. Loss of source is determined by a time-delayedundervoltage relay. The auxiliary relay (27X) starts the transfer by tripping the mainbreaker, and closing the tie breaker (after verifying the main breaker has tripped), butonly if no other protection function (50/51, 87B) has operated, and the other mainbreaker instantaneous undervoltage relay has not operated. The general arrangement ofthis basic scheme is shown in Figure 5.0The allocation in the Matrix is as below.

File Edit I_nfet DOVICe Miew OblioNi WindoW Help6

mmiifit LJLEN Indications and commands only Zj Nflift., ~ ~ ~ iiK~lhfbornatibn source Destinhgion

NM Diflat text. T9 e Bl F C BO: s C

I==E..............................................................~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~........27I=9 OA VLE O -Hi

= .......................... .....TOCHS_r2 M 4 52 b 7..9.0112114SP 1 2H 4_ B 7L = 1011 1213 1415l -sP W ~~~~~~~~~~~~~~~~~r p SPIP= - m_[I tt1 ___E__=__E__=__

O~~__F~uh_F~~e ______________Tie_Trp___ UT_

l ~~~~~~~~ ~ ~ ~~~~~~~~TieCIo-se.OUT| x|||l~~~~~~~~~~~~~~~~~~~~~o e . ...OUT I L L _Pro Trip .............~~~~~~ pOUT |X |U

27~~qAIVolt ________________r___e_ Ir_ S

meI re .... ..... - _

Den_ 1nee:.

I_~~~~~~"- ''''''''''

satist__iciilltil

SeFPbihtguret ......."1-H.0 H 0

The PLC programming capabilities of a numerical feeder protection relay allow asimplification of a main-tie-main automatic transfer scheme. The control logic, includingauxiliary devices, is in the programmable logic software of the relay. This method leadsto lower installed cost: fewer physical devices, less engineering and drawing, and lesswiring is required. The control logic is implemented in software, making customizationsimple. Field modifications do not require physical changes and the associated design,drawing, and wiring changes. Testing is simplified: testing of the scheme logic can beperformed with a bench test before actual wiring takes place. Commissioning tests needonly verify the limited switchgear or breaker wiring involved.

1 Prot Trip

Trip Tie Bkr!RelayfTri p Pro Trip

TB7 Tripl ~~~~~~~~~~~~~~~~~TripM21-1

i b 1s1~o Prot TripiI 27 Trip ,A uALto Tfr GoI

No M-2 Trip iTie BkrTOCbb

Close Tie Bkrhl-1 Open jI

D-Tfrr Mode (43IMI)

Fu]nctionl K(ey M'anual1 IModle (43M)

| 14~~~~~~~~oProt Trip L

CClose B-1t|P?ermnissiveII

C1ose Busn ~~~~~~~~~~~~~~~~closeM-1

7SJ63 Relay i

Figure 7.0

Performing a Main-Tie-Main automatic transfer scheme over Ethernet IEC 61850 withOvercurrent Relays has many benefits. As showed in the previous timing example theEthernet is faster than the copper connections. Because the scheme is performed insoftware, the number of auxiliary devices, the engineering design time, drawing time,and amount of wiring is reduced, lowering the first cost of installation. Performing thescheme in software also makes the scheme easy to customize and modify in the field.Field modifications are complete with auto-documentation from the relay. Test andcommissioning is simple, with a graphical test report that precisely documents elementoperation, and allows easy troubleshooting of misoperations. Result: lower operatingand maintenance costs.

Conclusions

Using the new lEG 61850 standard parallel hardwiring between devices can be replacedby Ethernet LAN. The capability for GOOSE message configuration gives the samedegree on security and surpasses timing of copper connections. Implementation of thisstandard leads to real cost reductions while,lt the same time the open functionality isincreased.

References[1] Norbert Schuster Reverse Interlocking Implemented by means of IEC 61850 GOOSEPraxis Profiline IEC 61850 July 2005[2] C. Holga, G. Wong, IEC 61850: Open Communication in Practice in Substations2005.[3] H. Schubert, G. Wong. IEC 61850 goes into practice", Modern Power Systems, April2004[4] A. Apostolov, D. Tholomier, S. Richards, Protection and control applications based onDistributed Analog Values as defined in IEC 61850. 59TH Annual Georgia TechProtective Relaying Conference April 27-9, 2005 Atlanta GA.[5] Holger Schubert, Project OCIS, aims and first results, an accompanying project to thestandardization of communication networks and systems "IEC 61850"[6] Robert W. Smeaton, Editor, Switchgear and Control Handbook, 2nd Edition,McGraw-Hill, Inc., New York, NY; 1987[7] 7SJ63 Multi-Functional Protective Relay with Local Control V4.2 Instruction Manual,Siemens Power Transmission & Distribution, Inc., Raleigh, NC; 2000[8] DIGSI 4 Device Configuration Manual, Siemens Power Transmission & Distribution,Inc., Raleigh, NC; 2000[9] DIGSI 4 CFC Manual, Siemens Power Transmission & Distribution, Inc., Raleigh,NC; 2000[1 0] Implementing the 47/47N Function in a 7SJ62/7SJ63 Protection Relay, TechnicalNote PRTN-0005A, Siemens Power Transmission & Distribution, Inc., Raleigh, NC;2001[11] Switching Command Interlocking in a SIPROTEC 4 Baycontroller, Technical NotePRTN-0006A, Siemens Power Transmission & Distribution, Inc., Raleigh, NC; 2001

72