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SCADA is often used in “ruggetized” environment such as power plants, manufacturing or remote weather stations. LSS [Line Sharing Switches] and SLSS [Substation LSS] (also known as Polling Controllers) are designed to monitor eight or more devices such as electronic (digital) meters, digital fault recorders or locators, alarms, capacity (energy management) devices and other IED [Intelligent Electronic Devices] via one or more leased or switched telephone lines. In addition, SLSS can provide secure encrypted data communications at data rates of 300-38,400 BPS.
A SCADA RTU [Remote Terminal Unit] performs remote control and monitoring, protective relays provide protection, strip charts record metering (historical) data, meter-dials display volts and amps and control handles and annunciator panels provide local control and monitoring. SCADA IED [Intelligent Electronic Device] replaced mechanical relay switches with computer microprocessor-based devices often called a PLC [Programmable Logic Controller]. IEDs support GUI [Graphical User Interfaces] which provide for more detailed, effective and versatile reports.
The IEDs also support other data protocols such as PPP [Point-to-Point Protocol], Ethernet, IP [Internet Protocol], Frame Relay, Wi-Fi [Wireless Fidelity] and other WAN [Wide Area Network] protocols as well as vastly increases data rates from 1,200 BPS [Bits Per Second] to MPBS [MegaBits Per Second]. Two primary means of data communications are used in multi-device called Point-to-Multi-Point SCADA systems. In a Polled or Master/Slave system, the master polls data (i.e., sends and receives data) to each slave in sequence. The slave unit responds to the master only when it receivers a request. This is called the half-duplex method. Each slave unit has its own unique address for identification. The other option is the Interrupt system or RBE [Report By Exception] system. In this instance, the Slave detects a limit change (to high/low or other value). The Slave then checks to determine that there is no other traffic on the network by analyzing the voltage of the transmission carrier – also called CS [Carrier Sense]. If no voltage is found on the transmission line, the Slave transmits its information to the Master. The SCAD [Supervisory Control And Data Acquisition] protocol consists of four parts.
That is, the SCADA protocol is a variable-length packet with up to 256 bytes of data with an optional Tuple carrying an additional 256 Bytes of user data. The SCADA protocol consists of four parts.
- DA [Destination Address] sent from the supervisor by the back end (monitoring device) after the supervisor requests it from the back end. The bits in this field represent a unique identifier. The first bit may also represent whether the message source is a supervisor or a controller. This allows the supervisor to know if it communicating with one of its own or other devices. While there is no current need for controllers and supervisors to communicate with each other, such functionality might be required in the future. The source address will occupy 8 bits, which allows 256 devices. If first bit device specification is used, 128 controllers and 128 supervisors can be used.
- SA [Source Address] filled in by the back end and the same length as destination addresses.
- ML [Message Length] specified in one byte and representing the number of bytes in a message’s body. This means that a message body may be up to 256 bytes long. An option is have longer messages the 256th byte is a length indicator for the subsequent tuple section of message. If a receiver finds a message to be 256 bytes long, it knows that the last byte must contain further length information, and thus that the actual message is more than 256 bytes long.
- MB [Message Body] up to 256 bytes of data unless with a tuple.
One of the applications for SCADA is for use in an electrical power grid. Like twisted-pair copper telephone lines and coaxial wire CATV systems, the copper-wire electrical power grid is a network capable of high speed data transmission. AC [Alternating Current] is modulated at 50/60 CPS [Cycles Per Second] allowing for data transmission at higher frequencies. The electric power grid consists of many different voltages such as:
- EHV [Extra High Voltage] – 400-1,000 KV [Kilo Volts]
- HV [High Voltage] – 35-400 KV
- MV [Medium Voltage] – 1-35 KV
- LV [Low Voltage] to 1 KV
Note: Others sources note that HV is defined by some as above 30 KV, so this is a general guideline, not an absolute one. As a footnote, many LAN switches and Wi-Fi APs use inline (data and power over the same wire) VLV [Very Low Voltage] at 48 VDC. As with any data transmission network, noise from lightning, short-circuits (accidental grounding), electric motor (electro-magnetic radiation), poor grounding and other causes increasing data errors and data loss. The Wi-Fi data transmission is connected to a router/switch for connection to an IP-Internet Protocol network or to an ethernet LAN switch via copper/fiber wire. The router/switch is in a separate environmentally-protected housing which may be located up to 100 feet away. Here at the power substation (switching center) the data transmission devices is “injected” (connected) on to the MV power transmission network via Wi-Fi. The Wi-Fi data signal is injected onto the MV distribution line, where it propagates (sent away) from the substation. Data repeaters regenerate (rebuild) the signal every 2,000 feet along the MV feeder until the termination point. Each repeater is also a Wi-Fi access point, providing access service to servers or the Internet to any Wi-Fi device within range. This could be used for Wi-Fi-on-the-road service along a highway.
At the house transformer where voltages are stepped down (reduced) from 13,000 or less volts to house voltages of 240/120 volts, the data “extractor” also operates via Wi-Fi or 802.11b. Wi-Fi operates at 2.400-2.483 GHz up to 11 MBPS-MegaBits Per Second at 100-150 feet with the same frequencies as cordless telephones and microwave ovens. Security is additional. Operational experience with Wi-Fi is rather new. Called Hotspots Wi-Fi AP-Access Points are nearly everywhere. Wi-Fi is an unregulated FCC-Federal Communications Commission RF-Radio Frequency with neighbors able to access each others Wi-Fi with little restriction or security. That is, security is added by the Wi-Fi provider, the Wi-Fi-AP device, the user or both. OFDM [Orthogonal Frequency Division Multiplexing] is a modulation technique used in PLC, in which data is sent using very closely spaced sub-carriers. This use of many carriers spreads the energy out across a very wide spectrum permits the signal to carry high data rates without interfering with radio signals using the same frequencies. In addition, OFDM technique makes the signal very tolerant of noise and reflections found on all power lines. OFDM modulation techniques are also used by the Wi-Fi (IEEE 802.11g) in the 2.4GHz radio spectrum. Over short distances, Wi-Fi data rates can be as much as 54 MBPS. OFDM works in well situations where noise from many different and simultaneous sources such as electrical motors (water fountains, soda machines), electrical spikes (power surges and lightning strikes), lighting switches, and others can cross the entire frequency spectrum on the power line.
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