SCADA & Remote Control Equipment

A Remote Control Centre (RCC) is set up near the Traffic Control Office on each Division having electric traction, working in close liaison with traffic control. The RCC includes the main control room, equipment room, Uninterrupted Power Supply (UPS) room, Remote Control Laboratory, and Battery Room — it is the nerve centre of Traction Power Control.

Two generations of equipment have been used at RCCs. FMVFT equipment (a stronger, mainly relay-based system) was used for all electrification schemes prior to 1980 and is now outdated. SCADA equipment, based on state-of-the-art computer technology, has come into use after 1980 — with fast growth in computer-based equipment, newer types with enhanced capabilities are regularly introduced.

What Is SCADA / Supervisory Control?

Supervisory Control is generally defined as a form of remote control comprising an arrangement for selective control of remotely located units, by electrical means, over one or more common interconnecting channels. In its most complex form, referred to as Supervisory Control and Data Acquisition (SCADA), it may control all generation, transmission, and distribution over a wide geographical area from one centralized location. Supervisory Control is a collection that provides an operator at a remote location with enough information to determine the status of a substation or generating station and to cause actions/operations to take place at that station without being physically present.

A Brief History

From 1900 to the early 1920s, many varieties of remote control and supervisory systems were developed. One of the earliest forerunners of modern supervisory control was a system designed by John Harlow in 1921, which automatically detected a change of status at a remote station and reported it to a control centre. In 1923, John Bellamy and Rodney Richardson developed a remote control system employing an equivalent of the modern 'Check-before-operation' technique to ensure validity of a selected control point before actual control was initiated. The first logging system was designed in 1927, monitoring information from a remote location and printing any status change together with time and date. At this stage, given the limited choice of components, all such systems were electro-mechanical.

Why Install Supervisory Control?

The usual reason is to provide system operators with sufficient information and control to operate the power system, or part of it, safely, securely, and economically. Before deciding to install a SCADA system, four factors must be clearly defined: who the users of the data are (operators for controlling the system, maintenance department for maintaining power supply equipment, management for decision-making, system designer for designing a better system); what data is available/necessary (a judicial balance between available finances and identification of all potential users and their requirements); what the user requirements are (the operator's need for timely, filtered, real-time data comes first, since almost all other users need the data only for post-mortem reporting); and what type of display is required (visual — VDUs, mimic diagram boards, strip-chart recorders, lights; recorded — hard copy from the logger; audible — bell/hooter to call operator attention).

Communication Configuration, Media & Techniques

Communication configuration is dictated by the number of RTUs, number of points at RTUs and required update rates, location of RTUs, communication facilities available, and communication equipment/technique available. The configuration used in Railway SCADA is party-line, one-on-a-few: the master station communicates with RTUs via a shared party-line communication channel, requiring a unique address for each RTU on the channel — sharing the master station's communication logic and the communication channel itself.

Communication Media

• Metallic cable pairs — as on date, of interest along with fibre optic cable. Little new technology; one pair can be used for simplex or half-duplex transmission at speeds up to 2400 bits/second. Must be protected against induced voltage surges/lightning by devices such as gas-tube protectors, and protected from ground-potential difference near high-voltage substations using neutralising transformers.
• Fibre optic cable — made of three basic components: a long fibre of pure glass/plastic (permits light travel from transmitter to receiver), and the receiver (converts light signals back to electrical signals, analog or digital). Repeaters convert light to electrical, amplify, and convert back; splicing devices allow minimum-attenuation travel between fibres.
• Power line carrier — NOT used in Indian Railways.
• Satellites — NOT used in Indian Railways.
• Microwave — in some sections of Indian Railways, a dedicated microwave channel at carrier frequency of 18 GHz has been provided for communication purposes.
• Leased circuits — an available option, listed alongside the above.

Advantages of Optical Fibre Communication

• Total immunity to electromagnetic interference — a prime advantage to electric power utilities.
• Very wide available bandwidth in the visible-light region compared to metallic cable pairs.
• Cross-talk between cables is impossible.
• Unaffected by lightning and electrical storms.
• Highly flexible and small physical size.
• Unlike a metallic cable, 'shorting' of a fibre optic cable cannot lead to invalid control signals or start a fire in a hazardous environment.
• No tapping or wave-dropping is possible without physically cutting the cable and coupling in.
• Long life expectancy.
• Very high speed of data transmission, limited principally by the capability of the multiplexing equipment.

Disadvantages of Optical Fibre Communication

• High cost.
• More repeaters required compared to metallic cable pairs.
• Transmitters and receivers are required to convert electrical signals to light and back — short-run cables would not be cost effective unless advantages like large bandwidth outweigh transmitter/receiver cost.
• Special connectors are needed to align fibres and require training to install.

Communication Techniques

Information may be conveyed by changing signal amplitude (Amplitude Modulation), signal frequency (Frequency Shift Keying), or signal phase (Phase Shift Keying). If information flows in one direction only, this is Simplex transmission; if in both directions but not simultaneously, Half-Duplex; if simultaneously in both directions, Full-Duplex. In Railways, Half-Duplex transmission is used. From an economy viewpoint, a single communication channel is used for many pieces of information — this is Multiplexing, with two basic versions: Frequency Division Multiplexing (each piece of information over a dedicated part of the channel — up to 25 sub-channels at 120 Hz spacing per voice-grade channel, beyond which an impractical number of channels would be needed) and Time Division Multiplexing (each piece of information transmitted as part of a serial digital message over a separate span of time, demultiplexed by the receiver).

Data transmission rate is measured in bits per second or baud — these units are not always synonymous. Baud is defined as 1/length of shortest signalling element in seconds (e.g. shortest element 0.833 milliseconds gives a signalling rate of 1200 baud/changes per second). Bits/second = Baud x bits/baud, where bits per baud is the amount of information (bits) per change. Two basic modem types: Asynchronous (uses separate timing sources like crystals at each end; message must be frequently resynchronised in short blocks; efficiency relatively high for short messages needing quick synchronisation; limited to 1800 bps in most applications, usually FSK type, low cost) and Synchronous (transmits a synchronising clock signal along with the data so the receiver stays precisely synchronised; permits very long messages and high data rates without falling out of sync; generally 2400 bps to 1 Mbps, higher cost).

Communication Message Formats

Transmission between master and remote using TDM techniques requires serial digital messages that are efficient (information bits transmitted / total bits transmitted), secure (ability to detect errors caused by channel noise), flexible (different amounts/types of information transmittable on command), and simple to implement in hardware/software (minimum logic, memory, storage, and speed of operation).

Every message divides into three parts: Message Establishment (provides signals for the receiver to select the correct message at the correct time, from hundreds of other possible messages and channel noise), Information (provides the coded data to be sent, allowing the receiver to decode and utilise it properly), and Message Termination (provides security checks so no errors in the message fields are accepted, and denotes the end of the message — a pre-defined number of check bits are transmitted and compared at the receiver; if identical, the message is accepted, otherwise retransmission is requested).

A typical asynchronous message format example: 8ms pre-transmission mark (to condition modem receiver for sync bits), 2-bit synchronisation (a space followed by a mark), 4-bit RTU address (allows a receiver to select the message addressed to it from all RTUs on the party line), 8-bit function code + 12-bit data (the information field — for remote-to-master messages, additional following messages also carry data in the RTU address/function code spaces, so 24 bits of data transmit — two 12-bit analog values or 24 bits of device status), a 5-bit BCH security code (allows detection of most message errors, triggering retransmission), and a 1-bit end-of-message mark (allows another message to immediately follow).

Information Transfer — Master-to-Remote Control (Select-Before-Operate)

Since operating the wrong device or receiving a bad control message can have severe consequences, additional security is required for control — provided via a sequence of messages called the 'select-before-operate' sequence: Remote-to-Master Checkback Message (Function Code | Control Address | Set Point), Master-to-Remote Execute Message (Function Code | Control Address), and Remote-to-Master Execute Acknowledge Message (Function Code | Control Address). Function code specifies the operation to be performed by the RTU; control address specifies the device/set-point to be controlled; set point provides the value to be accepted by the RTU. A remote-to-master checkback message is derived from the RTU point-selection hardware to verify the RTU has correctly interpreted the control selection. A master-to-remote execute message is transmitted only upon receipt of a proper checkback message. A master-to-remote execute acknowledge message is a positive indication that the desired control action was initiated. Prior to transmission of the above sequence, a control operator performs a similar select-verify-execute-acknowledge sequence via their own control console.

Batch data transfers from master to remote (e.g. parameters for report-by-exception, parameters for locally-controlled devices) use: Master-to-Remote Control Message, Remote-to-Master Ack Message, Master-to-Remote Control Batch Data Transfer Message (Function Code | Data Address | Data), and Remote-to-Master Ack Message. The first two messages prepare the RTU to receive the longer-than-normal message; the third transmits the data; the fourth indicates correct receipt at the RTU. Special security precautions are required if a party-line communication channel is used, so RTUs do not decode a batch data transfer message not meant for them.

Remote-to-master data transfer is accomplished with one basic message sequence, using variations in field definitions for different data types: Master-to-Remote Data Request Message (Function Code | Data Identification) and Remote-to-Master Data Message (Data Unit 1 | Data Unit 2 | Data Unit...). Function code specifies the type of data to be transferred by the RTU; data identification identifies the amount/type of data requested by the master station. On each message transmitted by the RTU (except messages containing only current data), it is necessary to retain the transmitted message in an RTU buffer, so that if the master station does not receive it correctly, a retransmission can be requested — otherwise the information would be lost.

SCADA Equipment at the RCC

The SCADA equipment at the RCC is called the Master Station, while the equipment at the controlled station is the Remote Terminal Unit (RTU). Underground telecommunication trunk cable transmits signals between the RCC and controlled RTUs — three pairs of conductors (one for 'Send', one for 'Receive', one spare) are made available for remote control operation from this cable; optical fibre cables are used in most divisions today instead of copper, and microwave communication is sometimes used.

• Isolating Transformers: to limit build-up of longitudinal induced voltage from 25kV traction induction effects, isolating transformers are provided on the cable circuit at intervals of 10 to 15 km. The cable is tapped at the RCC and each controlled station, with 3 pairs of conductors terminated on a terminal board; an isolating transformer of impedance ratio 1120:1120 is provided at the tapping point.
• Repeater Stations: voice-frequency repeaters are provided at intervals of 40-50 km to boost the signal and make good the attenuation. Repeater gain is about 20 dB, with an equaliser to compensate up to 0.02 dB/kHz/km — depending on distance, repeater gain is set so signal level at any point en-route stays within prescribed limits. Suitable surge arrestors are provided inside the equipment room where the cable lead is long; the metallic sheath of the lead-in cable is kept insulated from the switching station's earth system to prevent induction effects, with the insulated conductor led alone into control panels. LT 240V auxiliary transformer neutral earthing and RC equipment earth are kept separate and distinct, NOT interconnected — the switching station structure should also be solidly bonded to the track rails by two independent connections.

Master Station Equipment

Hardware Configuration: a dual main micro-computer/mini-computer system — one main, one hot standby — is provided at the master station, each interfacing with its own front-end processors (if any) and modems for RTU communication, plus the man-machine interface equipment. Each system has its own console, hard and floppy disks with drives. Watchdogs monitor system health; if one computer system fails, the standby takes over automatically. Two data-logging printers (one online, one standby) are connected to the same online computer system — if one printer fails, the other automatically takes over.

Man-Machine Interface (Work Stations): two work-stations, one for each of two operators, each with two semi-graphic colour VDUs and a keyboard (both functional keys for frequently-repeated operations and alpha-numeric keys for data/text input), are provided at the RCC. Both work-stations connect to the same online computer and meet these requirements: (i) normally control pre-defined and physically demarcated sections; (ii) in case of complete outage of one work-station, all its functions can be transferred to the healthy work-station so normal operations continue; (iii) normally one VDU is online and the other standby at each work-station, but both VDUs of each work-station can be online simultaneously if needed — one for viewing the telecontrol diagram, the other for alarms/trend curves/histograms/other data.

Mimic Diagram Board (MDB)

A mimic diagram board and its associated mimic driver, provided at the RCC, depicts the traction power supply diagram, indicating the energize/de-energize condition of sub-sectors of the catenary and the status of interrupters and CBs at TSS & FP, SSP, and SP. Unlike the conventional MDB used in RCCs prior to SCADA's introduction (where all control operations were carried out from the MDB itself), with SCADA all control operations are carried out from the keyboard(s) at the work stations — the MDB's sole purpose is to give the operator an overall view of the traction power supply system. Its size is therefore much smaller, and the intention is to eventually do away with the MDB altogether since the system can be viewed on the VDUs.

Annunciations

A controlled station's 'Remote Station Defective' condition is annunciated by an LED at the Master Station. The following annunciations by LEDs are also provided on the MDB: Main System 'ON', Standby System 'ON', Main System Defective, Standby System Defective, Main UPS Failed, Standby UPS Failed, UPS battery low (below 90% of nominal voltage), and 415V AC 3-phase supply to UPS failed.

• SCADA Software: the operating system used is suitable for multi-user, multi-tasking, networking, and real-time applications.
• VDU Display: application software supports a large number of versatile semi-graphic coloured displays for issuing telecommands, blocking/deblocking controlled points, viewing alarm listing, event listing, or special functions. Calling any VDU display is by simple keyboard operation with pre-defined options for order/manner of display calls.
• Transmission and Coding System: the master station equipment normally scans continuously all RTUs in a pre-defined cyclic sequence to update equipment status, alarms, events, and measurands. Exchange of information takes place on interrogation by master followed by reply from RTU. The communication technique is based on Digital Address Time Division Multiplexing; every data exchange follows a well-defined transmission protocol, and each transmission contains sufficient parity check bits to detect transmission errors.
• Historical Data Storage: the SCADA system is designed to store historical traction power supply data for one year, including all alarms/events of controlled stations and system alarms, plus day-wise storage of average feeder current/voltage, daily maximum demand, maximum and minimum feeder voltage on the 25kV side, total operations of feeder protective relays (OCR, DPR, WPC), CB trippings, and maximum/minimum OHE voltage on both sides of the neutral section. A memory capacity of 40 MB is provided for this purpose in the hard disk, with provision for further expansion as required.
• UPS and Batteries at RCC: a dual stand-alone UPS system of adequate capacity supplies 240V, 50Hz single-phase to the SCADA system at the Master Station. Both UPS units work in parallel sharing the load; if one fails, the entire load is automatically taken over by the healthy UPS without affecting system operation. If both UPS fail simultaneously, SCADA equipment load is directly connected to input mains through a static switch without any break. A single battery with adequate Ah capacity is provided with both UPS to provide 2 hours of supply to various equipment in case of failure of the 415V AC 50Hz input supply.
• Remote Terminal Unit (RTU): microprocessor-based, and includes its associated digital input/output modules, alarm input modules, analog input modules, watchdog, transducers, memory modules, interposing relays, summation, CTs, power supply unit(s), surge arrestors, and other items necessary for its proper working.
• Storage of Events and Alarms: all changes (one or more) in the status of CBs/interruptors/motor-operated isolators and alarms occurring between consecutive pollings are stored by the RTU until reported to the station — eliminating any loss in reporting to the master station due to intermittent channel failures or other reasons. In other words, no event is ever lost without being reported to the master station.

SCADA Equipment Capacity (Typical, per TSS/SP/SSP of a double-line section)

The number of controlled stations (TSS, SP, SSPs) controlled from an RCC depends on the section's requirements. Speed of Transmission and Update Time: communication between master station and RTUs operates at a suitable transmission speed not exceeding 600 baud. Normal polling time for updating status, alarms, and measurands is not more than 4 seconds for 20 controlled stations, and not more than 6 seconds for 30 controlled stations — actual timings may vary slightly depending on the communication protocol adopted and system design. The RCC should ideally be in the middle of the electrified section if the number of controlled stations exceeds 30, so scanning on either side is possible and update time is kept to the minimum.

• Modems: modulator/demodulators for master-RTU communication utilise Frequency Shift Keying (FSK) modulation, with send and receive functions independently programmed as required. The modem works satisfactorily up to an input signal level of -45 dBm. It also incorporates a necessary amplifier with a minimum gain of 30 dB to compensate for signal variation at different points in the system; suitable attenuation is provided within the amplifier to adjust the level through a trimpot/rotary switch, with output signal level adjustable between 0 and -30 dBm in steps of 1 dBm.
• Special Features in SCADA Equipment: (i) tripping of bridging interruptors on under-voltage at SP in extended feed condition; (ii) inter-lock release request facility for circuit breakers/interruptors control at boundary post (guidelines per RDSO letter No. ETI/PSI/RC/SPEC 19 dated 10.3.1989); (iii) control of shunt capacitor bank and monitoring of power factor; (iv) automatic localization of fault on OHE and isolation of faulty sub-sector.

Operation and Maintenance of RC Equipment

Duties of SSE (RC)

The SSE (RC) is the senior supervisor working under the control of DEE/AEE (TrD) and directly responsible for the proper operation and upkeep of RC equipment, which is vital for operation of the electric traction system. He must be thoroughly conversant with all technical details of the equipment under his control, and in particular shall perform the following duties:

1. Maintain the RC equipment at the RCC and controlled stations in accordance with prescribed schedules.
2. Keep close liaison with the S&T department as to the sound condition of cable pairs allotted for RC operation.
3. Measure periodically the levels of voice-frequency signals at controlled stations and arrange with the S&T department for correction, when required, at their repeater stations.
4. Keep in constant touch with the TPC on shift duty and ensure prompt rectification of defects reported in the RC system.
5. Ensure proper maintenance of UPS/battery sets for uninterrupted operation of the RC equipment and the standby generating set in the RCC.
6. Inspect the RC equipment at every controlled post once in two months.
7. Impart necessary training to staff under him in special techniques of maintenance of RC equipment as well as trouble-shooting.
8. Ensure that special instruments and tools provided for maintenance of RC equipment are properly cared for.
9. Keep a watch on stocking of spare parts and other stores required for RC equipment, and initiate timely action to recoup stocks.
10. Co-ordinate with SSE(PSI) and SSE(OHE), or territorial SSE, for manning the controlled posts in the event of persisting faults in the RC equipment.
11. Submit prescribed periodical returns on RC equipment to AEE(TrD) and Sr. DEE(TrD).
12. Keep superior officers fully informed of all important developments and seek their guidance when required.
13. Carry out such other duties as may be allotted by his superior officers.
14. Carry out minimum monthly inspection as per manufacturer's recommendations.

Operation of RC Equipment

The RCC is the nerve centre of the traction system from which full control over every switching operation on the entire electrified route is exercised. It should therefore be kept in perfect operating condition at all times — no one other than an authorized official shall at any time operate the equipment. The TPC shall carry out a lamp test at every change of shift, and once a day give a general check for all stations to ensure indications on the mimic panel are in order. Any defects observed in operation shall be brought to the notice of AEE(TRD); depending on the nature of the defect, the TPC shall take adequate precautions against mal-operation until the defect is rectified. Should RC equipment of a switching station fail completely, the failure shall be reported to AEE(TRD) and Sr. DEE(TRD), and arrangements made to man the switching station with trained personnel to carry out switching operations, observing the precautions prescribed in Chapter VIII (Breakdowns). The TPC shall once a day contact by telephone the operators of each grid sub-station from which traction power supply is obtained, to ensure communication facilities are intact.

Investigation of Failures by SSE

As with any other equipment, every failure of RC equipment should be separately registered, investigated, and rectified — with a brief note in the failure report of the action taken, along with classification and finally pin-pointing the exact cause of failure. The failure record should be analyzed every month and any special steps required taken to overcome the trouble and prevent recurrence. A history sheet showing faults that have occurred on different items of equipment will assist in carrying out detailed investigation of recurring troubles, in consultation with the manufacturers of the equipment.

Maintenance Schedule for RC Equipment

The specific maintenance instructions issued by the respective supplier of SRC/SCADA systems should be observed, and any changes therein may be decided in consultation with the manufacturer. For batteries used in the Remote Control Centre and the Remote Terminal Units (RTUs), the instructions in Chapter II for fortnightly and quarterly maintenance shall be applicable. SCADA systems on Indian Railways are supplied by various manufacturers — although the systems differ, basic features are similar; for details of operating instructions and maintenance, the operating/technical manuals of the makers should be referred to.