Deploying SCADA Systems

5.0 Deploying SCADA Systems

There are many different ways in which SCADA systems can be implemented. Before a SCADA or any other system is rolled out, you need to determine what function the system will perform.  Depending on whether you are a utility company or a telecommunications provider, you have a number of options in creating your systems.  There may be a need to employ different methods that are complimentary to each other. 

The way in which SCADA systems are connected can range from fiber optic cable to the use of satellite systems. The following sections will present some of the common ways in which SCADA systems are deployed.  We will also look at their advantages and disadvantages.   

5.1 Twisted-Pair Metallic Cable 

Twisted-pair telecommunications cable is the most popular medium used by utilities and has existed in its present form for many years. The cables are essentially the same as those used by the Telephone Company and contain a number of pairs of conductor.  

Aerial cables would be more appropriate for installation in the utility’s service area since the Utility may own a large number of distribution poles from which the cables could be suspended. The smallest aerial cables can be self-supporting, whereas large aerial cables have to be attached to supporting wires (messengers) by lashing wire. Table 5.1 shows the Twisted-Pair Cable advantages and disadvantages.

Advantages	Disadvantages
·         No licensing, fewer approvals ·         Existing pole Infrastructure ·         Economical for short distances ·         Relatively high channel capacity (up to 1.54 MHz) for short distances	·         Right-of-way clearance required for buried cable ·         Subject to breakage ·         Subject to water ingress ·         Subject to ground potential rise due to power faults and lightning ·         Failures may be difficult to pinpoint ·         Inflexible Network Configuration

Table 5.1: Twisted-Pair Advantages/Disadvantages [8

5.2 Coaxial Metallic Cable 

Coaxial cable is constructed of a center copper conductor, polyvinyl chloride (PVC) insulation, a braided or extruded copper shield surrounding the center conductor and PVC insulation, and a plastic jacket cover. Coaxial cable can transmit high frequency signals up to several MHz with low attenuation compared to twisted pair wires used for telephone service. Methods of installation used for existing systems in Europe and the USA are underground, direct burial, overhead, and on existing power line structures.

Services usually supported are voice, data, and interoffice trunking. Table 5.2 shows the Coaxial Cable advantages and disadvantages.

Advantages	Disadvantages
·         No licensing, fewer approvals ·         Existing pole Infrastructure ·         Economical for short distances ·         Higher channel capacity than Twisted- Pair Metallic ·         More immune to Radio Frequency (RF) noise interference the Twisted Pair Metallic	·         Right-of-way clearance required for buried cable ·         Subject to breakage ·         Subject to water ingress ·         Subject to ground potential rise due to power faults and lightning ·         Failures may be difficult to pinpoint ·         Inflexible Network Configuration

Table 5.2: Coaxial Cable Advantages/Disadvantages [8]

5.3 Fiber Optic Cable 

Fiber optic technology has improved considerably since its inception in 1970. The technology has improved to the point where commercially available fibers have losses less than 0.3 dB/km. Losses of this magnitude, as well as the development of suitable lasers and optical detectors, allow designers to consider fiber optic technologies for systems of 140 km or more without repeaters. 

Optical fibers consist of an inner core and cladding of silica glass and a plastic jacket that physically protects the fiber. Two types of fibers are usually considered: multi-mode graded index and single-mode step index fiber. Single-mode fiber supports higher signaling speeds than the multi-mode fiber due to its smaller diameter and mode of light propagation. Communication services usually supported by optical fiber include voice, data (low speed), SCADA, protective relaying, telemetering, video conferencing, high- speed data, and telephone switched tie trunks. Optical fiber cables have similar characteristics to twisted-pair communications cables in that aluminum tape or steel-wire armors and polyethylene outer jackets can protect them. However, the inner core is constructed to accommodate the mechanical characteristics of the fibers. Typically, the fibers are placed loosely in semi-rigid tubes, which take the mechanical stress. Special types of fiber optic cables have been developed for the power industry. One type of fiber cable is the Optical Power Ground Wire (OPGW) that is an optical fiber core within the ground or shield wire suspended above transmission lines. Another type of optical fiber cable is the All-Dielectric Self-Supporting (ADSS) cable that is a long-span of all dielectric cables designed to be fastened to high voltage transmission line towers underneath the power conductors. A Wrapped Optical Cable (WOC) is also available that is usually wrapped around the phase conductor or existing ground/earth wire of the transmission or distribution line. In the Utility’s case, aerial fiber optic cable can be fastened to the distribution poles under the power lines. Table 5.3 shows the Fiber Optic Cable advantages and disadvantages.

Advantages	Disadvantages
·         Immune to electromagnetic interference ·         Immune to ground potential rise ·         High channel capacity ·         Low operating cost ·         No licensing requirement	·         Novel technology, i.e. new skills must be learned ·         Expensive test equipment ·         Inflexible network configuration ·         Cable subject to breakage and water ingres

Table 5.3: Fiber Optic Cable Advantages/Disadvantages [8]

The cost per meter of fiber optic systems is expected to continually decrease. The cost of single mode fiber optic cables is now less than multimode fiber optic cable because of the increasing demand for single mode fiber. Conversely, the multimode fiber optic has limited distance and bandwidth characteristics. 

The fiber optic terminal equipment is simpler and generally less expensive than microwave equipment. Optical transmitters can be either light emitting diodes (LEDs) or laser diodes. They operate at 850, 1310, or 1550 nm wavelengths, depending on the application. Many optical terminals have been developed for the telephone industry for large numbers of channels. There are now a number of products specifically designed for power utilities. These are low capacity terminals that feature surge withstand capabilities and special channel units for tele-protection signaling. 

Parameters that influence the choice of the type of optical cable to be used are:

•        Overhead cable can be OPGW, ADSS, or WOC

•        Underground cable can be duct cable (light, medium, or heavy duty), ADSS for use in a duct, or direct burial cable with armor jacket

5.4 Power Line Carrier

Power Line Carrier (PLC) was one of the first reliable communications media available to electric utilities for critical communications channels that could not be subjected to the intolerance and unreliability of leased (common carrier) telephone circuits. PLC uses the power transmission lines to transmit radio frequency signals in the range of 30 kHz to 500 kHz. The physical security of this communications is very high since the power line carrier equipment is located within the substations. PLC systems are used to provide voice, telemetry, SCADA, and relaying communications on portions of the 220/230 kV, 110/115 kV, or 66 kV interconnected power transmission network.  

Digital PLC technology is a relatively new technology. Power lines and their associated networks are not designed for communications use. They are hostile environments that make the accurate propagation of communication signals difficult. Two of the biggest problems faced in using power lines for communications are excessive noise levels and cable attenuation. Noise levels are often excessive, and cable attenuation at the frequencies of interest is often very large.  

The cost of PLC will probably increase at a greater rate than inflation because of decreasing demand. Communication transmission capacity of Single Side Band (SSB) PLC cannot be increased without purchasing a second or third PLC Radio Frequency (RF) channel at the same cost as original terminal equipment. Some cost can be saved by sharing dual frequency Traps, Line Tuning Units and coupling equipment. Digital PLC can be increased from one channel to three channels within the same RF bandwidth. Table 5.4 shows the advantages and disadvantages. 

Advantages

Disadvantages

•        Located where the circuits are required •        Equipment installed in utility owned land or structures •        Economically attractive for low numbers of channels extending over long distance •        Digital PLC has capacity for three to four channels (e.g., two voice and one high speed data) •        Analog PLC has capacity for two channels (one voice and one “speech plus” low speed data

•        Not independent of the power distribution system •        Carrier frequencies often not protected on a primary basis •        Inherently few channels available •        Expensive on a per channel basis compared to microwave (normally, over four channels) •        Will not propagate through open disconnects

Table 5.4: Power Line Carrier Advantages/Disadvantages [8]

5.5 Satellites 

The use of satellites has been investigated for a number of years. The satellites are positioned in geo-stationary orbits above the earth’s equator and thus offer continuous coverage over a particular area of the earth. Satellites contain a number of radio transponders which receive and retransmit frequencies to ground stations within its “footprint,” or coverage, on the earth’s surface. A network facility on the ground tracks and controls the satellite. Earth stations are comprised of an antenna pointing at the satellite, a radio transceiver with a low-noise amplifier, and baseband equipment. Satellites use both the C-band and the Ku-band. Very Small Aperture Terminal (VSAT) technology has advanced to the point where a much smaller antenna (down to about one meter) can be used for Ku-band communications. This has resulted in the Ku-band being preferred for sites with modest communications requirements. VSAT technology is advancing steadily, and the capital costs have dropped substantially. Continual time-of- use charges must be considered in the use of satellite communications. Developments in this area should be investigated when making a decision on the use of this technology. Table 5.5 shows the Satellite system advantages and disadvantages.

Advantages	Disadvantages
•        Wide area coverage •        Easy Access to remote sites •        Costs independent of distance •        Low error rates •        Adaptable to changing network patterns •        No right-of-way necessary, earth stations located at premises	•        Total dependency on a remote facility •        Less control over transmission •        Transmission time delay •        Reduced transmission during solar equinox •        Continual leasing costs

Table 5.5: Satellite Advantages/Disadvantages [8]

5.6 Leased Telephone Lines

Leased telephone circuits have long been used to meet communications needs. Most organizations use standard telephones connected to the Public Switched Network (PSN) for office communications and for routine voice traffic to stations. Leased dedicated circuits are used for dedicated communication requirements, such as telemetry and SCADA. Wideband channels may be available for high speed data signaling. Circuit characteristics can often be conditioned for many other uses, including voice and various types of low and medium speed data. Table 5.6 shows the Leased Circuit advantages and disadvantages.

Advantages	Disadvantages
·         Small Capital Outlay ·         Maintained circuit quality ·         No communications expertise required ·         Adaptable to changing traffic patterns	·         Repair and maintenance is not controlled by the lessee ·         Circuits may not be available at some sites ·         Metallic links require protection against ground potential rise ·         Continual leasing costs

Table 5.6: Leased Circuits Advantages/Disadvantages [8]

5.7 Very High Frequency Radio 

The Very High Frequency (VHF) band extends from 30 to 300 MHz and is usually used by utilities for mobile radio, although point-to-point links have been implemented in this band. Advances in data transmission on mobile radios have been made, particularly for joint voice and data use, such as in taxi and police dispatching systems. Such systems could be used for maintenance vehicle dispatching. SCADA systems can use adapted VHF radios for communications; however a SCADA system would need exclusive use of the frequencies. Frequency assignments in this band are usually reserved for mobile services. Table 5.7 shows the VHF Radio advantages and disadvantages.

Advantages	Disadvantages
·         Frequency assignments available ·         Propagation over non-line-of-sight paths ·         Low cost radios compared to microwave ·         Less stringent waveguide and antenna requirements ·         Not dependent on power lines and common carriers ·         Greater field strength coverage patterns than UHF band	·         Low channel capacity ·         Low digital data bit rate ·         Limited transmission techniques available

Table 5.7: VHF Radio Advantages/Disadvantages [8]

5.8 Ultra High Frequency Radio 

The Ultra High Frequency (UHF) band extends from 300 to 3000 MHz. The bands typically considered for UHF radio are in the 400 MHz and 900 MHz range. 

Most of the suitable radio products for SCADA applications available in the U.S. operate in the 900 MHz frequency range. In the U.S., the Federal Communications Commission (FCC) regulates the use of radio frequencies and has designated the 928 to 952 MHz range specifically for use by utilities for data communication applications. These UHF systems can be Point-To-Point (PTP), Point-To-Multipoint (PTM), Trunked Mobile Radio, or spread spectrum systems. The PTM systems are also referred to as Multiple Address Radio Systems (MARS). Spread spectrum systems are the basis for many wireless applications including 802.11 a/b/g networks. These types of UHF systems are described in the following subsections. 

5.8.1 Point-to-Point 

Point-to-point communications is usually used for SCADA communications from a master station or dispatch center to individual substations. Radios used in the lower frequencies of the UHF band can be expected to have greater ranges, particularly for non- line-of-sight paths. Fewer studies have been carried out on path performance of fixed point-to-point links operating in these bands, compared to studies on line-of-sight microwave paths or on mobile radio coverage. Path analysis tends to use methods adapted from one of these areas of study and may give significantly different results. The best method to determine propagation losses is to make actual path measurements with suitable test equipment or modified radios. Table 5.8 shows the Point-To-Point UHF radio system advantages and disadvantages.

Advantages	Disadvantages
·         Frequency assignments available ·         Propagation possible over non-line-of- sight paths ·         Low cost radios compared to microwave ·         Less stringent waveguide and antenna requirements ·         Not dependent on power lines and common carriers	·         Low channel capacity ·         Low digital data bit rate ·         Limited transmission techniques available

Table 5.8: Point-to-Point UHF Radio Advantages/Disadvantages [8]

5.8.2 Multiple Address Radio Systems 

A Multiple Address Radio System (MARS) Radio System generally consists of one Master Station (usually Hot Standby, full duplex) transmitting over an omni directional, gain antenna in a 360 radiation pattern to fixed station remotes or slaves (usually Non Standby, half duplex) that receive the signals via a directional, gain antenna. The 400/900 MHz MARS Radio is a single channel system that communicates with each of its remotes or slaves in sequence. Services usually supported by MARS are SCADA, Telemetry/Data Reporting, and voice (on a limited basis). 

The security of a MARS system is high between stations, but is vulnerable at terminal stations in regard to the antenna and the terminal RF transmission lines. If security is a potential problem, the RF transmission lines can be placed in conduit, and the antennas can be ruggedized. The components of a MARS system have a long Mean Time Between Failure (MTBF) and friendly user maintenance features. The MARS system is usually configured for data transmission at 300 to 9600 baud, but can be used for voice transmission during radio system maintenance by locking the data signal out while voice is being transmitted. 

The channel bandwidth allowed by the FCC is 12.5 kHz, which limits the expansion and upgrade capability of the MARS systems. Table 5.9 shows MARS advantages and disadvantages.

Advantages	Disadvantages
·         Frequency assignments available ·         Propagation possible over non-line-of- sight paths ·         Low cost radios compared to microwave ·         Less stringent waveguide and antenna requirements ·         Not dependent on power lines and common carriers ·         Lower cost than Point-to-Point media	·         Low channel capacity ·         Low digital data bit rate ·         Limited transmission techniques available ·         Multi-point operation restricts data speed compared to Point-to-Point UHF or dedicated paths between stations

Table 5.9: MARS UHF Radio Advantages/Disadvantages [8]

5.8.3 Spread Spectrum Radio 

Low power spread spectrum radios3 are allowed to operate in the 902–928 MHz band, 2.4, and 5.3 GHz band without licenses. This has prompted the development of packet- type radio networks for data systems, which are appropriate for Digital Multiples System (DMS) applications, such as Distribution Automation (e.g., utilities). There are also systems offered in the 450–470 MHz band by a number of manufacturers. Each band can provide suitable signaling rates and characteristics for DMS communications, although the 900 MHz systems appear to have more advanced features at this time. Table 5.10 shows the Spread Spectrum Radio System advantages and disadvantages.

Advantages	Disadvantages
·         No radio frequency license required ·         Low cost equipment	·         Subject to interference from co-channel transmitters ·         No primary license status ·         Limited path lengths because of restrictions on Radio Frequency (RF) power output

Table 5.10: Spread Spectrum Radio Advantages/Disadvantages [8]

5.9 Microwave Radio 

Microwave radio is a term used to describe UHF radio systems operating at frequencies above 1 GHz, although multi-channel radio systems operating below 1 GHz are sometimes referred to as microwave systems. These systems have high channel capacities and data rates, and they are available in either analog or digital transmission technologies

. Analog transmission was the first microwave technology available. It is the most mature method of transmission. There have been a number of developments, which have affected the traditional balance between digital and analog technologies. On the analog side, direct-to-baseband analog channel units have been developed to ease the addition of channels to existing multiplexer equipment and to reduce the complexity of modifying the channel plan. On the digital side, products such as digital cross-connects and direct first order hierarchical level access to Private Branch Exchange (PBXs) have reduced costs further and added flexibility. There is also a growing demand for circuits with very high data rates, which can be transported much easier on the digital systems. New protocols and standards are being introduced for utility data communications that are more easily accommodated by digital carrier systems. Therefore, even for light communication traffic routes, digital is judged to be the more appropriate technology for new installations. Services usually supported by microwave communications include voice, data (low speed and high speed), SCADA, compressed video, protective relaying, telemetering, frame relay, Broadband Integrated Services Digital Network (B-ISDN), and fractional T1. 

In the lower part of the frequency range, microwave radios are designed in both point-to- point and point-to-multipoint configurations. The radios use similar technologies but are configured, operated and controlled differently. Point-to-point radio systems have dedicated transceivers and directional antennas at each end of a link. Point-to-multipoint radio systems have a common master transceiver with a non-directional antenna at the hub of a number of radial links. Point-to-point radios carry a fixed number of channels continuously. The equipment to which the channel interfaces are connected determines channel usage. For instance, they can be fixed, full-time data circuits interconnecting computer systems or telephone exchanges with usage determined by voice traffic. Channels are operational even if the circuits are idle. 

Point-to-multipoint radios operate like a local area network with a number of shared channels, which are used on a demand basis. Point-to-multipoint radios can operate in several modes:

•        Frequency Division Multiple Access (FDMA)

•        Time Division Multiple Access (TDMA)

•        Code Division Multiple Access (CDMA) 

FDMA is more suitable for analog radios because they have narrower bandwidths. TDMA and CDMA are more suitable for digital radios. Point-to-multipoint radios are more appropriate if network topology is in a star or tree configuration in which a number of terminal nodes have direct radio paths to a single central node and channel usage is not continuous. For linear configurations and continuous traffic, or bulk transmission over long distances, point-to-point radio is more appropriate. Table 5.11 shows the Microwave Radio advantages and disadvantages.

Advantages	Disadvantages
•        High Channel Capacity •        Transports high data rates •        Circuits added at low unit cost •        Independent from power lines and common carriers •        Future standardized high-speed networks •        Not vulnerable to “backhoe fading” •        Low right-of-way costs •        Simpler installation than cable technology	•        Line of sight clearance required •        Specialized test equipment and training required •        Frequency assignments sometimes unavailable in urban areas •        More expensive site development •        Limited capacity

Table 5.11:  Microwave Radio Advantages/Disadvantages [8]

New digital equipment requires very little maintenance compared to older analog systems. Many are overlaying the older analog microwave routes with fiber optic cable or new digital microwave equipment to supplement the older analog microwave system. Microwave communications are usually very secure physically since most microwave terminal and repeater equipment is located on utility premises.