384 IEEE Transactions on Power Delivery, Vol. 9, No. 1, January 1994

APPLICATION OF PROGRAMMABLE LOGIC CONTROLLEXS TO SUBSTATION CONTROL AND PROTECTION

J. G. GiIberl, PE G. R. Diehl, PE Senior Member, IEEE Member, IEEE

Pennsylvania Power & Light Company, Ahntown, PA

KEYWORDS

PLC, programmable logic controller, substation control, substation protection.

ABSTRACT

This paper describes the application of programmable logic controllers (PLCs) to control and protection of bulk power and regional supply substations. PLC requirements are discussed and the concept of "zones of control" is explained as applied to standard PP&L substation arrangements. The use of a peer-to-peer communication network between logic controllers and other substation equipment is also discussed. Details of an installation completed in spring, 1992, are presented, including experience acquired from power system faul ts.

INTRODUCTION

Essential factors in substation protection and control design are high availability and expedient return-to-service following a system failure. Electromechanical relay and control schemes lack self test and continuous monitoring features needed to speed return-to-service. Without continuous monitoring, availability of an electromechanical scheme is not really known until it is called on to operate. Self-monitoring features are provided in new electronic relay packages and in PLCs. The application of these features to utility protection and control schemes promises to improve knowledge of system availability and reduce troubleshooting times by identifying failures.

In the la te 1980s P P & L personnel respons ib le for commissioning, trouble shooting and repair of control and protection systems requested that existing protection and control schemes be reevaluated. The number of components and complex wiring to effect interlocking of schemes was becoming unmanageable for the manpower available. The design engineering group set out to develop protection and control schemes that used the least number of components, minimized wiring and provided the greatest self diagnostics. The result of that effort is explained in the following sections that detail the new substation protection and control design employed by PP&L at substations from 69 kV through 500 k V . T h e combina t ion of microprocessor re lays , programmable logic controllers and "smart" sequence-of- events reporting allowed the design engineering group to meet its goals at an affordable cost.

PLC APPLICATION TO CONTROL AND PROTECTION

Programmable logic controllers are modular, industrially hardened computers which perform control functions through modular input and output (110) modules. The modularity of PLCs allows the user to combine generic I/O modules with a suitable controller to form a control system specific to his needs. The operation of a controller is most simply understood by envisioning that it repeatedly performs three steps:

Reads inputs from input modules 0 Solves preprogrammed control logic 0 Generates outputs to output modules based on the

control logic solutions

93 WM 031-5 PWRD A paper recommended and approved by the IEEE Power System Relaying Committee of the IEEE Power Engineering Society for presentation at the IEEE/PES 1993 Winter Meeting, Columbus, OH, January 31 - February 5, 1993. Manuscript submitted August 25, 1992; made available for printing

PLCS are most Often programmed Using relay ladder logic. Ladder logic programming is a language well suited for use by relay engineers, since there is little difference between conventional elementary diagrams and ladder logic diagrams. The sequential repetitive nature of PLC ladder logic requires

December 28, 1992. careful thought to maximize speed of response, but presents many benefits compared to its hardwired equivalent. For instance, there are no "sneak circuits" in ladder logic.

PP&L's primary reasons for selecting PLCs over other control

0885-8977/94/$04.00 0 1993 IEEE

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alternatives were the generic nature of the hardware and the similarity of the programming software to present control logic design. PLC hardware and software health monitoring are a benefit in any application. Other benefits of applying PLCs may vary with the application, but generally include reduced wiring, fewer devices to purchase and mount, and the ability to a l ter logic rapidly a t minimal cost. A reduct ion i n engineering and drafting costs results as control logic and hardware designs are reused on subsequent projects.

It has been apparent for the past several years that these benefits would be realized by utilities, if PLCs could be applied to control and protection of substations. The PLCs would receive inputs from fault sensing relays, control switches, transducers, auxiliary contacts from breakers and other devices. PLC outputs would be wired to trip coils, communication equipment, and other substation equipment.

Until recently, the throughput (time from input to output) of PLCs prevented their application to time critical substation functions. PLCs were suitable only for applications such as control of automatic reclosing and carrier checkback schemes because PLC operation was too slow for tripping applications. The idea of applying PLCs in a substation was also a concern, s ince P L C s had been des igned for use i n indus t r ia l environments. These concerns are no longer val id . Controllers now available meet throughput and IEEE surge withstand requirements, can operate from a 125 Vdc power source, and are capable of directly tripping and closing breakers.

CAPABILITIES AND REOUIREMENTS

Traditional PLC ADDkations

Programmable logic controllers have been applied successfully to control industrial processes for approximately 20 years. These applications frequently require gathering large numbers of inputs to provide many outputs. The time between when input signals change and outputs reflect that change is of concern; but time delays of 50 or even 100 milliseconds are acceptable in many instances.

The environments where PLCs are installed are often very dirty, but heated rooms are generally available. Electrical transients, radiated and conducted, may be present. Power is generally provided at 24 Vdc or 115 Vac. Reliability is a concern, since failure to properly control a process may cause production losses, but seldom will failures endanger human life.

Utilitv PLC Reauirements

Requirements for utility protection and control of large substations differ in three significant respects from the

requirements above. The common utility battery voltage is 125 Vdc, relatively few inputs (i.e. 64) and outputs (i.e. 32) must interface with equipment specific to the utility industry; and time delay between input and output must be no greater than a few milliseconds for high speed power system protect ion applications. Delays o n the order of 10 milliseconds are acceptable for subtransmission substation tripping functions, but transmission substations often require 4-5 millisecond tripping from control schemes to maintain system stability.

There a re two ways to meet the ut i l i ty’s high speed requirements; one through software, and one through a split architecture hardware approach. The software speed increase is accomplished by forcing the logic controller to read time- critical 1 / 0 several times for each time lower-priority operations are performed. When changes in critical U0 are noted, the logic program jumps to small high-speed tripping subroutines. The drawback to this method is that the control logic is no longer clear to field engineers and technicians who have minimal computer language backgrounds.

PP&L chose the split hardware architecture because it was well suited to the split in requirements between high-speed tripping and lower-speed reclosing and auxiliary functions. In this approach, the tripping functions are loaded into a combination high-speed intelligent I /O module and the reclosing/auxiliary functions are loaded into the main controller. The high-speed module and main controller communicate over the rack backplane to exchange necessary information. The high-speed module used by PP&L is actually a self-contained programmable logic controller having sixteen inputs and eight outputs, which fits within one card slot of the conventional PLC rack.

The high-speed module is only used where required. Relatively low-speed operations such as 69 kV breaker trip control and lockout functions, transformer lockout functions and bus tripping and testing are handled in the main controller which has a throughput time of 8 to 10 milliseconds. This throughput time is roughly equivalent to the delay of a master trip relay. The throughput time is doubled to 16 to 20 milliseconds for bus faults, but remains comparable to the time for a conventional lockout. Only time-critical functions such as breaker tripping at 230 kV and above are performed in a separate high-speed module. Time delay from input change to output activation in this module is typically on the order of one millisecond.

Application of PLCs to 125 Vdc control systems continues to be a problem, both in supply power to the controller and in the interface of 110 to operating devices. PLC manufacturers continue to expand their offerings at 125 Vdc, but PP&L has not found all necessary components at the speeds required to be available. This issue is most easily addressed by the installation of a 24 Vdc battery system. By using modem

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absorbed electrolyte cells and selecting a PLC with low power requirements, the cost and size of the battery system can be kept small. The use of switching power suppl ies was considered, but abandoned as less reliable than a battery system. As a side note to the discussion on power supply, PP&L testing revealed a three-to-one rat io in power consumption between various manufacturer's PLCs. This was factored into the cost analysis when selecting a PLC supplier.

The interface of inputs and outputs to substation equipment is the second aspect in dealing with the 125 Vdc problem. Inputs were not a problem as they could be wetted at 24 Vdc to permit the use of readily available 24 VDC input cards. Most outputs could also be handled at 24 Vdc since many were connected to other PLCs, indicator lights, or the alarm system which are available with 24 Vdc inputs. The problem is that, even though 125 Vdc output modules are available, none can handle the high current requirements of circuit breaker and motor-operated switch trip and close coils. PP&L elected to use a 24 - 125 Vdc solid state relay as the interface device. These, commonly referred to as "hockey-puck" type relays, have been available for many years for 115 Vac systems and just recently have become available for higher voltage DC systems. PP&L has been working with various manufacturers to increase the number of suppliers for these relays. These relays can handle high currents for the times required to trip a breaker, can interrupt the DC current path and add less than 0.1 milliseconds to the operate time of a protective scheme.

Some control functions require that decisions based on analog values be made within controllers. Reclosing control through synchronism check is one example which requires knowledge of phase angle across an open breaker and the voltage on either side of the breaker. PLCs are capable of analog input, but do not sample at a rate that would permit processing AC waveforms. Commercially available power transducers are therefore used to transform the phase angle and voltage values into bipolar 0 to 1 milliampere signals with 100% overrange capability. PLC analog modules were not available to directly interface at these signal levels. "Normal" analog input values for PLCs are 0 - 20 or 4 - 20 milliamperes, or voltage input of 0 to +I-5 or 0 to + / - lo volts. PP&L worked with one manufacturer to modify an existing module for 0 to +/-2 milliampere interface. This is now a standard product with its own model number. PP&L strives to avoid "specials" and very much prefers any product purchased be part of a standard product line offering.

Although not generally used, it is possible to purchase power transducers with a 4-20 milliampere output; however, the method of achieving this output entails amplifying and offset t ing the low current signal. This increases the components in the transducer and masks the potential benefit of having a live zero signal.

REDUNDANCY VS. SEPARATION OF SCHEMES

Control system reliability and maintainability are of paramount concern. PP&L applies PLCs in a redundant configuration whenever protective functions are incorporated in the logic. Redundancy insures that critical PLC functions are always available and that individual PLCs can be removed from service without loss of control or protection functions.

Electromechanical control schemes operate under a separation of schemes philosophy. Schemes were independent such that a single component failure would not disable multiple protect ive control functions. This approach was not economically feasible or technically desirable in applying PLCs. PLCs are designed with a great deal more computing power than would be required by a single control function and the cos t of apply ing P L C s in th i s fash ion would be unacceptable. In addition, the expected savings in wiring and termination costs would be lost. The solution was to employ redundancy to replace separation of schemes, while maintaining the requirement that a single component failure could not disable multiple control functions.

There are various forms of redundancy offered by PLC manufacturers. PP&L chose to fully duplicate the control system hardware controller and I/O for every protective func t ion . T h e redundant cont ro l sys tems opera te independently and unsynchronized in an eitherlor control action arrangement. This architecture requires that primary and backup protective relays provide inputs to both of the redundant PLCs. Optical isolation incorporated into the PLC input cards provides the necessary separation of primary and backup. Wherever possible, isolated contacts are wired to each PLC. Single component failures in this control architecture cause no loss of control functions.

The only control functions operating off a single controller are SCADA and TCUL. Although important to the operation of the substation, these functions do not warrant the expense of redundant hardware.

Redundancy does pose its own set of problems. A method must be employed to prevent the output fuses of both redundant PLCs from opening for a short circuit at a common output point. An inadvertent screwdriver across a common indicator light may be all it takes to disable multiple outputs from both redundant units. PP&L chose to use self-protecting outputs on all its PLC systems. These outputs sense output current, voltage and transistor junction temperature, and shut an individual output down if any design parameter is exceeded. In addition, the module sends an alarm bit back to the main controller identifying which output has been disabled. Use of this type output prevents loss of multiple functions for a single failure.

The smart, self-protecting outputs do have a drawback. When

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Transmission Voltage Breaker Bay Control - Trip and close control of all breakers and motor-operated disconnect switches in a high-side breaker bay; single, double breaker or breaker and 1/2 arrangements. Interface with all protective relays for up to two lines.

the module containing these outputs is deenergized, the outputs exhibit a very low impedance, providing a feedback circuit. This circuit path is capable of reenergizing a powered down PLC rack. PP&L originally solved this problem by adding a single diode per output module to provide isolation of input power, but later decided to use an output test connector that incorporates diodes for every output, as shown in Figure 1. The first solution was functional, but did not prevent the energized PLC from lighting the field side output indicators of the deenergized PLC. The false indication could result in operator confusion.

MAINTENANCE AND TEST FACILITIES

A decision was made to provide special test facilities for the PLCs. All outputs from each PLC rack are run through a high quality, high density (56 pin) test connector. This allows testing and maintenance to be performed on individual logic controllers with minimal disruption to the station wiring. A PLC rack can be fully exercised after a hardware or software modification without risk of misoperation. In addition, removal of this test plug generates alarms which inform the local repair crew and the remote system operator that a PLC rack is out-of-semice. As already noted, blocking diodes are incorporated into the test connector assembly.

~

1.

2.

3.

4.

5.

ZONES OF CONTROL

The selection of Zones of Control is a tradeoff between maximizing the use of the controller’s computing power and minimizing the risk associated with combining multiple control functions in one package. In addition, careful selection of zone boundaries can minimize control logic interface between controllers, thereby minimizing wiring. PP&L found that the following Zones of Control provided the best utilization of the control hardware within acceptable risk parameters. These are applied, as applicable, to transmission substations of any arrangement used by PP&L.

DC suppLyl / FUSE REMOVED DlmE &Gas ByxmoD 10 OTlER PLC WULUUE

WO mCA1ca TO OTHER

H A R D W E PLC

PROTECTED I IouTRlr

TO OTHER

HARDWARE

RACK N2

Figure 1 - OUTPUT BLOCKING DIODES

Transmission Voltage Bus Tripping & Restoration Control - Interfaces with all bus protection relays, for one or both high-side buses, for tripping control. Also coordinates restoration testing and reclosing lockout following a bus trip.

Transformer Control - Control of the low side transformer breaker(s), high-side motor-operated disconnect switch and ground switch. Interfaces with transformer protective relays, and breaker failure relaying on both the high side and low side of the transformer.

Subtransmission Breaker Bay Control - Trip and close control of all breakers in multi-breaker line bay, or two single breaker-single line bays. A maximum of two subtransmission lines are controlled by this zone.

Subtransmission Voltage Bus Tripping & Restoration Control - Interfaces with all bus protection relays, for one or both low-side buses, for tripping control. Also coordinates restoration testing and reclosing lockout following a bus trip.

The functions of zones 1 and 4 are similar except that Zone 1 incorporates a high speed tripping module. Zones 2 and 5 differ in the way that the analog processing is handled and the division of responsibility between breaker bay and bus controllers.

Figures 2. and 3. show the Zones of Protection and Zones of Control for one of PP&L’s typical 230169 kV substation arrangements. In this arrangement, only four control zones are used since there is no high-side bus. Identical control zone programs are used for each of the two transformers, and similar reuse of control logic is employed for each low-side line bay.

Voltages Below 69 kV

PP&L believes i t i s not cost effective to use PLCs in distribution substations in the same way they are applied at transmission substations and, therefore, no thought was given to these substations when the control zones were developed. However, it does appear that PLCs may be effectively applied at distribution substations to replace single function

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Figure 2 - Zones of Control

electromechanical or solid state relays. For example, small PLCs could turn out to be cost effective replacements for reclosing relays. Also, the use of PLCs as supervisory remotes in non-PLC based substations of all voltage levels is under development.

INTERZONE COMMUNICATIONS

Communications between control zones is broken into two functional groups; those that perform tripping, and those that perform restoration or data gathering functions. The tripping functions are hard-wired between the zones. The restoration and data gathering functions are handled over a peer-to-peer communications network. The communications network selected introduces delays which vary with network loading and therefore was not suitable for high speed tripping.

Hard-wired Communications

The hard-wired communications can take the form of direct input to output wiring or can be arranged as a "trip bus" where many controllers' inputs and outputs are tied together in a bussing arrangement. Information on transformer trouble and line faults is passed between the high-side breaker bay controllers and the transformer controllers as direct input-to- output connections since these are the only controllers involved in this control action. Breaker failures and bus tripping actions require the simultaneous control action of many controllers. For this reason, breaker failure and bus trips are handled by creating hardwired busses.

Peer Network

The network used works by passing a token between controllers. If the token is lost it is regenerated automatically. The network operates at a data rate of 1 megabit per second over shielded pair cable. This high data rate and use of a

Figure 3 - Zones of Protection

"global database" feature makes information placed on the network by a PLC available to all other PLCs within one or two scans or cycles of the application programs. If necessary, data can be transmitted directly from one controller to another, rather than through the global database. The network permits all controllers to communicate with an operator interface (01) computer connected to the network. The 01 can send and receive data, monitor controllers' health to the card level, and program the controllers over the network. The network will both reduce interpanel wiring and simplify maintenance and troubleshooting.

ApDIication to Bus Triupinp & Restoration

Hard-wired trip buslpeer network operation is best explained by reviewing the communications associated with bus tripping and restoration. When a bus fault is sensed, all breakers on the faulted bus must be opened quickly to isolate the fault. Traditionally tripping by the bus relays was done by wiring the trip coils of every breaker to an electrically resetable lockout relay. All breaker trip coils could be wired to the bus protection PLC just as they were wired to the lockout relay, but substantial wiring can be saved by tripping the breakers through other PLCs already wired to the trip coils for other purposes. Reliable high speed operation is achieved by wiring a "trip bus" between PLCs. This trip bus is wired to an output of the bus protection PLCs and to an input of all PLCs which control breakers needed to clear a bus fault. When the "trip bus" is energized, the associated input of all breaker bay controllers is energized informing them to trip the breaker(s) associated with the faulted bus. As long as a "trip bus" remains energized, breaker closing is prohibited. Bus testing is performed over the peer network as follows:

1, A "block reclose" command for breakers attached to the faulted bus is sent out through the global database to all controllers.

2. The preprogrammed bus testing breaker is enabled to test the bus as won as the bus controller deenergizes the trip bus.

3. Based on the outcome of the bus test, either of the following may occur:

0 The test is successful and a message is sent out through the global database to reenable automatic reclosing of the bus breakers.

0 The test fails, the "trip bus" is reenergized and a message is sent out disabling automatic reclosing of the bus breakers. .This command message will then be maintained until the bus is reenergized manually or by SCADA and holds for a suitable time period.

LOCAL METERING AND SCADA

Local metering consists of multiplexed intelligent displays mounted on the control panels. Voltages, currents, megawatt, and megavar flows are scaled to engineering units within the display units. Normally each quantity is displayed for two seconds before proceeding to the next quantity; however, stop/start, forward and reverse push buttons permit operators to freeze the display on any selected quantity. All scaling and display functions are performed entirely within the display uni ts so that local metering i s not dependant o n PLC operation.

In addition to providing local metering the display units provide analog quantities to the SCADA PLC through the peer network. The displays cannot be connected directly to the peer network at this time, but they are tied to the peer network through a "bridge multiplexor. " PP&L routinely installs SCADA remotes in it bulk power substations to provide remote monitoring and control. The traditional SCADA remote is eliminated in PLC-based substations by collecting data and performing control over the peer network. A dedicated controller collects the data for transmission to the SCADA master computers. Control commands received from the computers a re sent over the peer network to the appropriate PLC.

PP&L is presently investigating whether the master/slave protocol built into the PLCs and the intelligent displays is suitable for use as the SCADA master-to-remote protocol. The PLCs are presently not capable of communicating with the SCADA master computers by the protocol applied with conventional remotes, so a protocol converter is used to make the connection.

PROJECTS IN ENGINEERING

PP&L has one PLC control based substation in-service, one

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50% complete and six in engineering, all using the zones of control described above and all using similar control logic. The first of these substations was placed in service in April, 1992. Port ions of a second substat ion which is being retrofitted also began operation at this time. The other projects have in-service dates ranging from Fall of 1992 to Spring of 1995. PP&L has made a commitment to PLC-based control and protection based on the potential benefits and the success of our first projects. Further experience with these projects will be reported in a follow-up paper.

JACKSON SUBSTATION

Jackson substation was the first PP&L PLC controlled substation to be placed i n service. It i s located near Stroudsburg, Pa. in the northeast portion of the PP&L power system. This station consists of one 138 kV breaker, two 75 MVA transformers with low side breakers, two 69 kV busses, and two 69 kV breaker-and-1/2 bays supplying three 69 kV transmission lines. Initially the station is supplied by a single 138 kV transmission line. A second 138 kV supply line will be added in late 1992.

Details of the control and protection system are as follows:

Number logic controllers: 14 Type of controllers:

Modicon 984-385 Modicon B984-101 (High Speed Tripping)

Programs and (number of controllers): - 138 kV Breaker Tripping (in 2-B984-101s)

(138 kV controls built for 230 kV operation) - 138 kV Reclosing and. Auxiliary Functions (2) - 138/69 kV Transformer Isolation and Restoration (4) - 138169 kV Transformer Tap Change Under Load (1) - 69 kV Bus Tripping and Restoration (2) - 69 kV Breaker and 1/2 Bay Control (4)

0 Number of intelligent panel meters: 8 0 Communication Network: MODBUS PLUS

- number of nodes connected: 18 - number of bridge multiplexors: 3 - token rotation time: 14 milliseconds

0 24 volt battery size: 430 Ah / 8 hour rate 0 125 volt battery size: 420 Ah / 8 hour rate 0 Number of sequence-of-events recorder points monitored:

448 Operator interface: - Processor: 20 MHz 80386, ISA bus - CRT: 19" color VGA

0 Number of control panels - Triplex: 2 - Duplex : 8 - Simplex: 4

- SCADA (1)

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A functional block diagram of the system is shown in Figure 4.

controls correctly tripped, reclosed, and finally went to lockout. Finally, on August 4, 1992, lightning resulted in a simultaneous fault on two 69 KV circuits. Both lines tripped and successfully reclosed on the first attempt.

Commissioning testing is simplified both by PLC control and by the use of "package type" electronic relays, especially when combined with sequence-of-event reporting. The reduction of control wiring associated with PLC control and package relays reduces the test time confirming point-to-point wiring connections. Computer-aided relay setting and testing reduced the time to commission the relays. Control scheme testing was greatly simplified through the use of the networked substation 01. The PLC programming and troubleshooting software presents a live "elementary diagram" of the control scheme. This elementary diagram can be monitored on the 01 during testing to ensure that the correct inputs were received and that all aspects of the control response were correct for each test. In addition, the control schemes can be exercised from the 01 by forcing logic elements to desired states.

The sequence-of-events recorder used for substation monitoring is an invaluable tool for start-up commissioning. It was used to monitor the alarms generated by each test, verify the completion of each control response and verify control response times. This was invaluable in performing final checkout of the control logic and in providing a basis of comparison for future operations.

POWER SYSTEM FAULTS

Jackson has experienced faults on four occasions. On May 2, 1992 lightning struck a 69 kV line. Relays correctly sensed a fault and tripped the associated circuit breakers. The logic controller initiated a reclosure, which was successful. A second instance occurred on June 23, 1992 when lightning struck the supply 138 kV line. The control at Jackson correctly followed the right-of-way reclosing of the remote breaker. The third operation occured on July 14, 1992 when a contractor caused a tree to come in contact with a 69 kV line and a supply 138 kV line under construction. Again the

COST

As already noted, the primary motivation behind application of PLCs and microprocessor relays is to incorporate self monitoring and trouble shooting features into controls in order to reduce return to service times and increase availability. In most cases these benefits are achieved at about the same cost as electomechanical solutions, but significant hardware cost savings may also be realized in some cases. For example, the TCUL control cost, including development, is about one half that of the three transformer standard cabinet used previously, as indicated in the table below.

No of transformers cost cost paralleled Conventional PLC

2 $30,000 $22,000 3 $41,000 $22,000

RELJABJLLITY

Factors which affect functional reliability include hardware design, software, interconnection wiring complexity, and mean time to repair, assuming that mean time to repair includes the time required to detect failures. Electromechanical relay control system designs historically have incorporated primitive manual monitoring systems to indicate relay health, such as lamps to indicate coil continuity. Often protective relays and auxiliary relays were applied with no monitoring facilities because of the difficulty and cost of incorporating monitoring. Consequently, failures of protection and control systems were not detected until called on to operate.

The combination of microprocessor relays and programmable logic controllers allows most of the protection and control system to be continuously monitored and alarmed. This

Figure 4 - Block Diagram

minimizes the time from failure to repair. Of course it is not feasible to continuously monitor the entire system. For instance PLC input and output circuitry is not monitored, but redundancy ensures correct operation for single failure. Sequence of events reports are reviewed whenever the protection operates to detect failures in a redundant system. A reduction in wiring also adds to reliability. Software reliability is an important aspect, but, unlike wiring, software which has been proven to be reliable can be copied repeatedly without introducing errors. Therefore, microprocessor-PLC approach is expected to have a higher availability than traditional electromechanical designs.

SUBSTATION SIMULATOR

PLC programs to be used in substations will be thoroughly tested during the engineering stage. PP&L has constructed a simulator in which a PLC programmed for substation control and protection is connected to another PLC which i s programmed to simulate substation apparatus such as breakers, motor-operated air breaks, load tap changers, carrier sets, etc.

The simulator was used to verify transformer tap changer under load (TCUL) control programs earlier this year prior to installation of PLCs in two substations. The PLC based TCUL operates under an entirely new control algorithm based on circulating VARS. Therefore, i t was essential to thoroughly test the logic through simulation. PP&L believes that the simulator will continue to be a valuable tool which will permit all but the most subtle problems in PLC control programs to be identified and resolved at the engineering design stage.

PLC DATABASE

PP&L uses a da tabase p rogram in the design and documentation of PLC systems. The database records PLC hardware and options installed, software setting, PLC-to-panel terminal block wiring, and any other relevant information. Job drawings containing this information are created using the database program. Production of these drawings does not require drafting resources; however, these drawings are used by drafting personnel to create panel wiring drawings.

CONCLUSION

Jef frey G. Gilbert, P. E. received h i s B.S.E.E. degree i n E l e c t r i c a l Engineering from Lafayette College i n 1972 and his M.S.E.E. degree f r o m Lehigh University in 1978. He has been employed in engineeringpositions by Pen nsy 1 vania Po we r and Light Company since 1972. From 1986 to present, he has been employed as a

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Senior Project Engineer i n Substation Engineering where his responsibilities include design of protection and control systems. Jeff serves on several working groups of the Power System Relay Committee. He is 4 senior member of IEEE and is a registered Professional Engineer in the Commonwealth of Pennsylvania.

Gerald R. Diehl. P.E, graduated with a B.S.E.E. from Pennsylvania State University i n 1978 and a M.S.E.E. f r o m Lehigh University in 1986. He is a registered Professional Engineer in the State of Pennsylvania. In 1978 he began employment with Pennsylvania Power & Light Company in the Relay & Control Section of Bulk Power Engineering. From 1988 to the present he has worked the in the Substation Engineering Section where he designs programmable control and alarm systems for regional and bulk power substations. During this time he has designed and implemented the wnes of control described in this paper, the software for PLC based SCADA and PP&Ls SER bused Alarm Management System. He is presently a Project Engineer in the Substation Engineering Section.

PP&L is committed to the use of programmable controllers in the control and protection of the upcoming generation of substations. The use of controllers interconnected by a peer network will reduce wiring leading to cost savings. Reliability and maintainability will be insured by redundancy of PLCs and special test facilities.

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DISCUSSION J. Pinto de 66(Technical University of Lisbon, Portugal): The authors have reported with a great detail an interesting experience in which PLC were applied to automate Substations. The main emphasis was oriented to physical issues, while the soft- ware was reported as having been written in relay ladder logic, be- cause of its similarity with the traditional diagrams. Going a step ahead, and giving up the ladder logic to evolve to other more sophisticated programming capabilities of PLC (eg. Grafcet languages), the possibility arises to rethink the functional capabilities of the PLC based control and protection of substations. The com- plexity of software may then appear as the most critical issue, but formal tools can be employed to keep it under control [Dl]. I would like to have some comment from the authors regarding how they in- tend to exploit the full capabilities of a software-based logic to en- hance the functionality of substation control and protection. As an additional comment, I would like to say that some utilities have already realized for a long time that PLCs are too expensive for dis- tribution substations. The inexpensive approach that was found for implementing software-based logic in such substations was to include it as pure software packages in already existing Remote Terminal Unities which provide the Input/Output environment [D2].

References: [Dl] - J.L. Pinto de S i and J. Damasio, “Coordination of Automatic Control Functions in Transmission Substations, Using Petri Nets”, EEE Trans. on PWRD, Vol. 7, Ne. 1, January 1992, pp. 262-268. [D2] - J.L. Pinto de Sa and S. Paiva, “A Multitasking Software Ar- chitecture to Implement Concurrent Switching Sequences Designed with Petri Nets”, IEEE Trans. on PWRD, Vol. 5, Ne. 4, October 1990, pp. 1766-1772.

Manuscript received February 25, 1993.

M. F. Fernandes (Electricidade de Portugal, SacavCm, Portugal): We were pleased to know that the use of PLC in Substation Control and Protection is now taking place in Pennsylvania Power & Light Company’s Transmission Substations.

As stated in our paper [1]-EDP SA (the company that runs the Portuguese Power Generation, Transmission and Electricity Dis- tribution Networks) has a consolidated experience in using this type of hardware since 1984. In fact, nowadays up to 45% of its transmission substations are equipped with PLCs, executing in real time complex automation functions and applied to different sizes of substations (from a few to up to 35 feeders-lines, buscouplers and transformers-and from 2 to 3 different voltage levels).

Due to the complexity, diversity and predictable evolution needs of the automation functions (undervoltage tripping, restoration after undervoltage tripping, delayed reclosing after protection line tripping, restoration after under frequency load shedding, etc.), special care has been taken in the selection of PLCs, since the very beginning. Namely, it has been a decision criterion to be able to program in Literal languages (not ladder logic) and to use “GRAF C ’ representation, due to the possibil- ity, thus created, of directly converting into program our func- tional specifications based in “mono-marked Petri Nets” (this decision was one of the key factors for the inexistance of software bugs after commissioning). CPU speed, multiprocessing and the existence of special software tasks with very short execution cycles (needed for the acquisition and pre-treatment of critical signals) were also important decision factors.

We came to the selection of high level industrial PLCs from TELEMECANIQUE (TSX60, TSX80, TSX67 and TSX87). These PLCs were submitted to tests and proved to conform to IEC surge with stand requirements and, thus, to be able to work properly in the electromagnetic environment of transmission substations. Until now the reliability has been very good [l].

Therefore, we would like to pose the following questions: 1. What specification method did you use for Control func-

tions? 2. Do you plan to evolve from ladder representation to other

(Literal, GRAF C, etc.) more adequate to provide direct soft- ware documentation and easier to trace in later software debug- ging and to make future expansions?

3. What was the manpower (man X month) for the first appli- cation (Jackson substation), concerning: software development, software integration and laboratory testing, on-site testing? What are the expected gains (for instance in development time and costs) for the following systems?

Reference [ 11 “Automation in Portuguese Transmission Substation: A

Pragmatic Approach,” 92 SM 389-7-IEEE Power Engi- neering Society 1992 Summer Meeting, Seattle, U.S.A., July 12-16, 1992

Manuscript received February 25, 1993.

J. G. GILBERT, G. R. DIEHL: The authors thank the discussers for their comments. The PLCs selected could have been programmed using a “GRAF C” type language. As already noted, we decided to use relay ladder logic because this permitted well tested interlocks and logic to be programmed with minimal risk of introducing errors. Our goal was to reduce inter-panel wiring and to provide improved hardware diagnostics. Only two new sequences were added because additional needs were not identified. PP&L does little switching, automatic or manual, at transmission substations on a routine basis. Sequences performed following faults are relatively straight forward and do not require the sophistication of Petri nets. Also, where throughput times are critical, such as for breaker tripping, ladder logic affords the engineer complete control over the implementation. The software package we used in writing the ladder logic provides documentation such as labels, network headers describing operation of the logic, and cross reference tables. This is supplemented by a document describing the overall logic. Consequently, we plan to continue to use ladder logic until there is a strong incentive to do otherwise.

Mr. Femandes notes that PLCs have been used in substations for many years for control functions. The authors are aware of other well established applications of PLCs to low speed control functions. However, except for work performed at Ontario Hydro at distribution voltages, we are not aware of utility application of PLCs in which the PLCs perform protective relay initiated breaker tripping and related functions such as distributed bus differential and breaker failure lockout at transmission substations.

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The authors do not agree that PLCs are too expensive for distribution substations. In fact, cost savings can be achieved by replacing conventional RTUs with small PLCs. The PLCs are commodity items, unlike custom RTUs, and once installed as an RTU substitute, can be programmed to perform automatic sequences and other functions. Although application of PLCs to distribution substations was not reported in this paper, PP&L is applying PLCs in its distribution substations as SCADA remotes. We would also note; however, that microprocessor based distribution relays are capable of performing automatic sequences such as automatic reclosing.

Design of hardware and software generally involves documenting functional requirements, developing specifications and a test plan and then proceeding with detailed design steps. Existing PP&L Engineering Instructions, drawings and file documents formed the functional requirements and specifications. S i ~ l a r l y , the "final" test involved the PP&L

Electrical Test Department conducting acceptance tests just as is done on every new substation. Testing was also performed during engineering to verify fimctionality , particularly where timing was critical.

Approximately 3 man-years of effort went into software development for Jackson. There was no incremental cost for on-site testing over electromechanical schemes previously applied at similar substations. The Jackson software was written for a generic breaker and one half arrangement and can be used at other breaker and one half substations without modification, except for changing a few settings in registers. The next two substations constructed after Jackson were placed in-service with less than budgeted costs. Costs lower than budget can, at least in part, be attributed to the reuse of "standard" software.

Manuscript received April 5 , 1993.