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1. INTRODUCTION
Complex and dispersed power systems necessitate large manpower resources for control, maintenance and management functions. Such resources may be reduced or employed more efficiently with the help of computer systems.
This section describes the basic interfaces, software and hardware necessary for transmission and distribution of power system supervisory control and data acquisition (SCADA). Programmable logic controllers (PLCs) and sub station bay controllers may be used for local automatic control functions.
These controllers are described together with a practical interlocking application example. The section also introduces traditional power line carrier communication and signaling methods. Communication via fiber optic links is mentioned separately in section 12. The section goes on to describe a centralized power transmission network control system and covers the very important subject of software management. Such management is essential if software development is to be achieved within quality, time and budget constraints.
2 PROGRAMMABLE LOGIC CONTROLLERS
2.1 Functions
Programmable logic controllers (PLCs) were initially developed for discrete control applications in machine and materials handling production engineering environments. The on-going development of PLCs for the control and monitoring of industrial systems has increased their capabilities from simple hard wired logic elements (NAND, NOR gates) to advanced functions using software-controlled microprocessors for piping and instrumentation diagram (P&ID) algorithms, floating point arithmetic, network communication and multiple processor configurations for parallel processing. Modern PLCs are capable of handling power system local control automation requirements.
IEC 61131 is rapidly becoming the internationally recognized standard for configuring PLCs. PLCs evolved to become bay controllers, which are basically items of relay grade hardware that are directly connected to instrument transformers (CT/VT) in a substation and have built in binary input and out put modules together with a large logic function library that can then be programmed like a PLC. The significant difference between a PLC and a bay controller is the fact that bay controllers are built with substation requirements in mind such as EMC, high making and breaking capacities, added relaying functionality etc.
2.2 PLC Selection
2.2.1 Control and Monitoring Specifications
The development of a control system may be divided into various stages as shown in the project development lifecycle diagram (Fig. 1). A management decision, based on timing and resource availability, is made as to the best stage to obtain competitive tenders for remaining design, supply and installation work from specialist contractors. The first step is to carefully detail the system to be controlled together with possible future expansion requirements. This initial description must carefully detail the hardware and software interfaces and addresses such questions as the physical location of devices, supervisory control connections, motor or actuator loads and physical enclosure protection.
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FIG. 1 PLC system development lifecycle.
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The second step is to define the operational control requirements in a concise and accurate descriptive form. At this second stage, it is essential that full consultation is made with operatives and maintenance crews as well as the engineers in order to ensure the correctness of the descriptions and definitions of the user's wishes. These descriptive control requirements are then converted in a particular format as a sequence of logical events.
A specification (sometimes termed Functional Design Specification or FDS) is next prepared for both the hardware and software. The hardware specification should cover the following points:
_ conformity requirements with any existing systems;
_ communication gateway (RS232/485, etc. or fiber optics glass/plastic) and associated 'protocols' (the protocol is the transmitting/receiving data exchange rules which govern the message format, timing and error checking);
_ the input and output devices to be connected either directly or via interposing accessories;
_ power supply requirements;
_ codes and applicable standards;
_ installation environment (enclosure protection, temperature, humidity etc.);
_ factory and site testing requirements;
_ documentation and quality assurance (QA) requirements.
The software specification provides a complete and definitive statement of what the control system has to do but not, at this stage, how to do it. It provides the basis for the system design and implementation and includes both the descriptive and logical sequences and functions taking into account:
_ functions to be implemented (Boolean or sequential, P&ID functions, maths etc.);
_ data exchanges (type of information per actuator or motor, analogue values, commands to be exchanged etc.);
_ complete input/output listing;
_ system software (redundancy or self-diagnostic);
_ support structure (programming tool giving access to PLC for on or off line testing and diagnostics);
_ factory and site testing requirements;
_ documentation and QA requirements.
A search is next made for the PLC system that is best fitted to these care fully defined needs at the most competitive price. The size of the PLC sys tem is determined from what tasks it is required to perform by defining the input/output requirements, the memory size and spare capacity. Other requirements include the piping and instrumentation (P&ID) loop control, floating point maths to perform the calculations and special functions such as time delays. This is the stage for selection from the various options for the most appropriate technical solution.
2.2.2 Technical Solutions
The options will comprise of a list of hardware and software components which will implement the functions specified in the system specifications.
Once the choice is made, detailed design of the PLC system follows. The various algorithm and logical sequences, time delays, fault treatments and data exchange tables used have to be validated through the detailed software design document. Such a document may use:
_ logic blocks or ladder logic in line with ISA Standards;
_ organograms if development is in the specific pseudo software language used by a particular PLC supplier;
_ IEC 61131_3, programming languages for programmable controllers.
The 'response time' of the PLC is the time it takes to translate a change on an input to effect an output. This is not the same as the 'scan time' which is only one of the response time elements involved. The response time takes into account:
_ the input/output update times;
_ the times to process counters, timers and mathematical instructions;
_ communication times if the PLC is part of a network control system.
A typical example would be a PLC scan of 1,000 instructions in 10 ms and a response time of 35 ms per 1,000 instructions.
2.2.3 Communication Links
Local automatic primary substation control will invariably involve more than one PLC. The integrated control system will require data to be passed from the switchgear to the associated PLC, from one PLC to another and also to the overall supervisory management system. Correct communication links are therefore the key to the fully automated system and will be the source of problems at the commissioning stage, if not properly defined. In the past manufacturers have introduced their own communications protocols and for mats such that a variety of software/hardware communication standards exist. It was due to this the standard organizations in Europe and North America started working on open standards. This work has culminated in a set of standards called IEC 61850. It is therefore essential for the user to define the communication standards to be used at the outset (or refer to the open standard) when preparing the specification and enquiry documents.
There are three principal communications network arrangements as shown in Fig. 2.
FIG. 2 Communication network arrangements. Star Ring Bus
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FIG. 3 Interface for remote control of power equipment.
Power supply Power supply battery Electrical power equipment Control of plant __ Analogue inputs Solid state input C and C power supply Digital I/O Mechanical Plant Control (PLC) Mechanical equipment Transducers LP/RTU selector Digital indication from plant Analogue indication from plant CT VT Plant terminal block C and C wiring interface IR Interposing relay Isolatable terminal blocks 110 V DC (max.) 4-20 mA DC 4-20 mA DC 4-20 mA DC RTU SCADA marshalling cabinet Controls and communications Volt free contact 48 V DC A V kW
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The ring and bus/optic star arrangements are the most widely used.
Twisted pair copper, coaxial and fiber optic cables link the network together with the choice depending upon transmission protocol used, quantity and rate of information being exchanged, length of circuits and cost for the particular application. Other special considerations for the system specification include:
_ system resilience requirements;
_ alarm reporting;
_ operator/graphics station;
_ electromagnetic compatibility;
_ documentation;
_ interface requirements.
FIG. 3 shows a typical switchgear control, metering and alarm inter face. In this example, a separate marshalling cabinet is proposed with jumpers between the switchgear equipment connections and the SCADA control termination blocks. This arrangement has the advantage of greatly simplifying testing and maintenance by allowing easy access to all the interface points in one cubicle. The disadvantage of the dedicated separate marshalling cubicle is the added expense, the space requirements and the introduction of additional connections into the system.
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FIG. 4 Single line diagram for PLC application example.
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TABLE 1 Voltage and Current Data for Example
Description Sense Range/Engineering Units Voltage incoming circuit A 4_20 mA 0_500 V Current incoming circuit A 4_20 mA 0_100 A Voltage incoming circuit B 4_20 mA 0_500 V Current incoming circuit B 4_20 mA 0_100 A
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TABLE 2 Digital Inputs for Example
Description Requirement Logic Mode switch circuit A Automatic 1 Circuit breaker A position Open 1 Circuit breaker A position Closed 1 Circuit breaker A condition Faulty 1 Mode switch circuit B Automatic 1 Circuit breaker B position Open 1 Circuit breaker B position Closed 1 Circuit breaker B condition Faulty 1 Mode switch circuit C Automatic 1 Circuit breaker C position Open 1 Circuit breaker C position Closed 1 Circuit breaker C condition Faulty 1
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2.3 Application Example
2.3.1 User Requirements
FIG. 4 is a single line diagram of a switchboard with two incoming
circuit breakers, A and B, and a bus section circuit breaker, C. In this
example the simplified user requirements are:
_ Automatic control for closing and opening circuit breakers A, B and C.
_ Remote monitoring of circuit breaker A, B and C positions.
_ Display of incoming circuit current and voltage.
_ Control modes to be either local manual or local automatic by PLC.
A brief introduction to the basic requirements for a remote control system is also provided in this example.
2.3.2 Input and Output Requirements
The analogue and digital input and output requirements are defined:
(a) Analogue inputs: The analogue inputs are described by standard loop current transducers as shown in TABLE 1.
(b) Digital inputs: The digital inputs are defined by logical '0' and '1' condition states. Some simplifications have been introduced into this example (e.g. no maintenance or earth positions for the circuit breakers have been introduced) since the purpose is to explain the basic design steps to be followed rather than describe a complex case (see TABLE 2).
(c) Digital outputs: While defining digital outputs, it is very important to consider their duty range. If they are expected to act on the process, i.e. operate a circuit breaker, the switching load and breaking loads may have to be considered or suitable interposing relays added externally (see TABLE 3).
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TABLE 3 Digital Outputs for Example
Description Requirement Logic Circuit breaker A Open command 1 Circuit breaker A Close command 1 Incomer A voltage display Binary-coded decimal (BCD) 4 digits Incomer A current display Binary-coded decimal (BCD) 4 digits Circuit breaker B Open command 1 Circuit breaker B Close command 1 Incomer B voltage display Binary-coded decimal (BCD) 4 digits Incomer B current display Binary-coded decimal (BCD) 4 digits Circuit breaker C Open command 1 Circuit breaker C Close command 1
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2.3.3 System Specification
Normal and abnormal operating conditions are defined together with any sys tem operating constraints in conjunction with maintenance, operations and engineering staff:
(a) Normal condition:
_ Circuit breaker changeover control is under automatic mode.
_ The left-hand side of the switchgear busbar is fed from incomer A with circuit breaker A closed and the bus section circuit breaker C open.
_ The right-hand side of the switchgear busbar is fed from incomer B with circuit breaker B closed and the bus section circuit breaker C open.
(b) Abnormal condition:
_ Power input failure or circuit breaker A faulty on incomer A. The whole busbar (both right- and left-hand sections) shall be fed from incomer B with the bus section circuit breaker C closed.
_ Power input failure or circuit breaker B faulty on incomer B. The whole busbar (both right- and left-hand sections) shall be fed from incomer A with the bus section circuit breaker C closed.
(c) Operating constraints:
_ Bus section circuit breaker C must not be closed when circuit breakers A and B are both in the closed position. Such a constraint may typically be due to fault level restrictions on the switchgear with the two incoming supplies paralleled.
_ When circuit breaker A or B or C is under local mode, the automatic control is disabled. This is a safety constraint.
_ When power supply is restored after failure on incomer circuits A or B, the system should remain in its current configuration awaiting operator intervention. This ensures positive action and status acknowledgement by operations personnel.
(d) Communications requirements between PLC and communications controller:
_ Master/slave configuration where the PLC/Bay controller local to the switchgear is the slave and the communications controller, which has intelligence and interfaces between the communications network and the PLC, is the master for remote control purposes.
_ Define the communications network protocol (e.g. 'Modbus, IEC 61850')
_ Define the serial link format (e.g., RS 485, optic bus glass _ plastic).
(e) Remote control operation:
Usually, the central control centre (CCC) is comprised of duplicated minicomputers as the central processor associated with various man/machine interfaces. These interfaces include such items as a hand-dressed (Fig. 5) or automatically updated (Fig. 6) mimic displays, operator consoles/key boards, visual display units (VDUs), data loggers, event recorders, telephone, public address and radio speech communications. The CCC acquires information from the communications controller or remote telemetry units (or 'remote terminal units') (RTUs) associated with interrogation scan. Each RTU has a unique address code and is accessed in turn for a given period of time when information requests or control signals may be sent and information received. In order to avoid large amounts of data overloading the system during a fault (e.g. a busbar fault would create a multitude of circuit breaker status changes, network load flow alterations, metering and alarm indications) information is prioritized. Further, 'front-end processors' are used for data acquisition in order to free the main computer for data processing.
2.3.4 Detail Design to Tender Enquiry Stage
From the foregoing a logic block diagram is next prepared as shown in Fig. 7. This should then be fully detailed into an overall descriptive and technical specification such that enquiries may be launched with different manufacturers to supply equipment for the particular application. FIG. 8 shows the PLC control cubicle for the Channel Tunnel 21 kV network automatic interlocking. This is designed to ensure that the UK and French unsynchronized Grid supplies are not paralleled inadvertently by incorrect switching sequences. The PLCs are in the top left-hand corner of the cubicle.
FIG. 5 Hand dressed conventional mimic panel. The panel consists
of individual engraved tiles to display the substation single line diagram,
switch position and key metering.
3 POWER LINE CARRIER COMMUNICATION LINKS
3.1 Introduction
The use of transmission lines as communications channels has obvious advantages to the electrical supply utility since it saves investing in additional dedicated communications radio, hard wire or fiber optic cable links.
System Control and Data Acquisition (SCADA) requires a communications network to transmit the information back to a central control centre (CCC).
There is a fundamental relationship and trade-off between the amount of information that may be transmitted over a given communications circuit, the speed of transmission and the bandwidth of the communications channel involved. The larger the bandwidth the faster a great amount of information may be transmitted. Hence, the bandwidth is the limiting factor for the signaling speed upon which the telecontrol system response times are based.
Power line carrier circuits operate at only a few hundred kHz carrier frequency with signaling speeds restricted as a consequence to approximately 600 Baud when a single (4 kHz) channel is shared with speech, and up to 9,600 Baud (analogue) or 28 kb/s (digital) on an equivalent data only channel. With modern digital equipment, it is possible to attain signaling speeds .64 kb/s using a good quality line (e.g. better than 30 dB signal-to noise ratio).
FIG. 6 VDU display of 132/21 kV substation single line diagram.
The subject is well covered by IEC Standards as listed in TABLE 4.
The data transmission is performed by modulating the carrier frequency using audio frequency shift keying with modem (modulator/demodulator) interface units. Higher carrier frequencies (and hence larger bandwidths and signaling speeds) are not possible because of the stray capacitance (and hence high losses and attenuation) involved in overhead line power circuits.
Telecontrol systems designed around power line carrier communication links are therefore specified with rather slow five second response times. The response time here is defined as the time between a change of state occurring at an outlying substation and it being announced at the CCC. This is one of the major reasons why fiber optic cable communications links are taking over from power line carrier-based systems. The other key reason is the immunity of fiber optic links from electromagnetic interference.
FIG. 7 Logic block diagram.
FIG. 8 Typical PLC control cubicle for switchgear interlock control.
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TABLE 4 IEC Standards for Power Line Carrier
Standard Description IEC 60353 Line traps for AC power systems IEC 60481 Coupling devices for power line carrier systems IEC 60495 Recommended values for characteristic input and output quantities of single sideband power line carrier terminals IEC 60663 Planning of (single sideband) power line carrier systems
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FIG. 9 Amplitude modulation of a radio frequency carrier and associated
double side band full carrier (AM) or single sideband reduced carrier
(SSB) spectrum.
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3.2 Power Line Carrier Communication Principles
3.2.1 Modulation
Power line carrier systems amplitude modulate the carrier frequency. Full amplitude modulation (AM) has a frequency spectrum of sidebands symmetrical about the carrier frequency as shown in Fig. 9. These sidebands contain all the information being transmitted and the carrier frequency is only the bearer of the messages. It is therefore possible to achieve savings in transmitter power without degrading signal performance by reducing the power of the carrier frequency and by deleting one of the sidebands. This is known as single sideband (SSB) transmission. The carrier is not fully sup pressed because it is used to synchronize the remote end receiver with the corresponding transmitter. The transmitter and receiver circuits therefore tend to be slightly more complex than those used for normal broadcast AM transmission because of the filtering and accurate synchronizing involved.
The lower ranges (B30_200 kHz) of carrier frequencies are used on long transmission lines and the higher ranges (B200_500 kHz) on shorter lines.
This helps to offset the attenuation effects of long lines with high frequency transmission. Computer simulations may be used to optimize the best frequency band to employ on a given overhead line taking into account interference from any adjacent circuits.
Each power line carrier path can carry one audio frequency (AF) channel.
This requires a minimum bandwidth of some 4 kHz. The lower B2 kHz end of this base band is often reserved for speech which requires a bandwidth from approximately 300 Hz to 2,000 Hz. It is possible to use the speech band for teleprotection, thereby freeing up bandwidth for data transmission and the channel is often used as a telephone system for the electrical supply utility.
Dialing pulses may be transmitted by shifting a pilot signal frequency and detecting the shift pattern at the receiving end. An override facility is normally also provided for emergency/maintenance purposes whereby the telephones are connected directly via a front panel jack socket into the speech circuits.
The remainder of the channel bandwidth, 2,000 Hz to B3,480 Hz, is available for telecontrol, teleprotection, and telegraph transmission using frequency shift keying (FSK). This form of modulation has many advantages over on-off keying of the carrier frequency and provided that the wanted signal (mark or space) is slightly stronger than any interfering signals, the information will be correctly received. The main difficulty is that the use of automatic gain control (AGC) is very limited and the time constant must be short. This is because the mark and space (logical '0' and '1') frequencies are only separated by a few tens of Hz and may fade independently of one another. A strong mark may be followed by a weak space especially under power fault conditions. A pilot signal, added in the spectrum outside the audio base band (e.g. at 3,600 Hz), is therefore used for supervision of the power line carrier channel and regulation of the receiver automatic gain control (agc). The Baud is the shortest single signal unit in a signaling code and may be expressed as the reciprocal of the time of the shortest signal element.
For example, if the shortest signal element were 20 ms in length, then the data transmission speed would be 1/0.02550 Bauds. The bandwidth of the telecontrol channel is determined by the frequency shift speed. A 200 Baud telecontrol channel shifting 690 Hz occupies 360 Hz of bandwidth and the frequencies used are selected from CCITT standard recommended channels as described in IEC 60481 and 60663.
Power line carrier schemes are used in conjunction with overhead line distance protection direct intertripping/blocking or permissive intertripping/ blocking as described in section 10. It is, of course, essential that such signals are correctly transmitted and received over the very transmission line that the protection scheme is attempting to protect from the consequences of a prolonged fault. During the fault noise will be generated that could degrade the teleprotection signal. Therefore the power line carrier teleprotection signal is boosted to maximum power and all other signals may be disconnected (speech and telecontrol) in order to improve the reliability under fault conditions.
3.2.2 Circuit Configurations
It is not usual to find power line carrier installations on distribution lines at voltages ,36 kV. This is because such lines tend to have many tee-off points which would attenuate the signals and necessitate the installation of many power frequency rated filters or 'line traps'. Also short power lines may employ pilot wire protection and the telecontrol system requirements may be able to use spare pilot cable cores.
The high frequency carrier signal is coupled to the overhead transmission line via high voltage coupling capacitors of value around 5,000 pF. These act as a low impedance (few hundred ohms) at carrier frequencies but as an open circuit at power frequency (B0.6 MO at 50 Hz), thus isolating the radio equipment from the power equipment. In addition, coupling filters and transformers are necessary to match the power line carrier transmitter output impedance to the overhead line and thereby ensure maximum power transfer.
The carrier frequencies must not be effectively short circuited to ground through earthing switches at substations or through the neutrals of power transformers. Each power line carrier overhead line transmission circuit must therefore be effectively isolated at radio frequency from the substation bus bars, transformers and switchgear.
FIG. 10 Power line carrier coupling arrangements. Phase-to-phase coupling; Phase-to-earth
coupling
FIG. 11a CVT and line trap installation arrangement.
FIG. 11b Qaboos 132 kV substation, Oman-OHL incomers with surge arresters,
CVTs/ line traps and cable sealing ends.
This is achieved by 'line traps' which are parallel inductance and capacitance (L, C) tuned circuits. These line traps are inserted in series with, and at the end of, the transmission line to act as high impedance at the carrier frequency and to prevent such frequencies entering the substation busbars. The line trap coil has a low impedance at 50 Hz in order to minimize power frequency losses. Surge diverters are connected across the tuned circuit to pre vent damage against surges. The traps are specified to carry rated current and to withstand short circuit conditions. A diagrammatic representation of this commonly used arrangement is shown in Fig. 11a.
FIG. 10 shows phase-to-earth and phase-to-phase coupling arrangements between the power line carrier radio frequency equipment and the power frequency overhead line. Phase-to-earth coupling requires only half the equipment necessary for the phase-to-phase method. FIG. 11b shows a Middle East 145 kV substation overhead line bay with the incoming gantry and Power Line Carrier line traps mounted on CVTs associated with the distance protection scheme. The CVTs are used to couple the power line carrier signal to the overhead line. If a power system fault occurs on the phase being also used as a teleprotection or telecontrol channel the power line carrier signal will be considerably degraded and an assessment has to be made as to the security of the system under these conditions. For double circuit transmission lines it is possible to arrange the power line carrier protection inter tripping for one circuit to be transmitted over the adjacent circuit. In this way the teleprotection channel does not signal over the actual line it is protecting.