Communications schemes for protection and control applications are essential to the efficient and reliable operation of modern electric power systems. Communications systems for power system protection and control are evolving, spurred on by the rapid increase in demand for real-time information, the availability and decreasing costs of new communications systems, and the elimination of some traditional communications systems. Traditional protection and control communications systems, such as dedicated metallic pairs and analog microwave radio, have been virtually eliminated because of the need for more communications bandwidth and the demand from more lucrative uses. At the same time, increasing demands on existing power system infrastructure are creating increased demand for faster fault-clearing and special protection and control schemes to keep power systems stable. Evaluating the new protection and control communications opportunities to determine if they are suitable requires an in-depth understanding of both the protection and control application requirements and the communications medium characteristics. This presentation provides the basis for this discussion. Suggested reading for this discussion includes the following SEL technical papers: Integrating Remotely Located Substations Into SCADA Systems: A Case Study Using Commercially Available Satellite Internet Service Providers for SCADA Communications, by James W. Rice of Plumas-Sierra Rural Electric Cooperative and Nicholas C. Seeley of SEL. Automation at Protection Speeds: IEC 61850 GOOSE Messaging as a Reliable, High-Speed Alternative to Serial Communications, by Nicholas C. Seeley of SEL. Digital Communications for Power System Protection: Security, Availability, and Speed, by Edmund O. Schweitzer, III, Ken Behrendt, and Tony Lee of SEL. Applying Radio Communication in Distribution Generation Teleprotection Schemes, by Edmund O. Schweitzer, III, Dale Finney, and Mangapathirao V. Mynam of SEL. 1
Protection and control communications applications include the following: High-speed line protection and transfer trip schemes. Remote breaker, switch, or generator status monitoring for autoreclosing schemes. Distribution automation and dynamic network reconfiguration. Distributed resources monitoring and control. Wide-area protection and control for islanding detection, remedial action, and special protection schemes. Synchrophasor measurements are a growing part of this function. Supervisory control and data acquisition (SCADA). 2
Desirable attributes for protection communication include the following: Low latency to achieve tripping with the least delay possible. Communication that is unaffected by fault-generated noise (when the communication is needed the most). Control applications do not typically need the same speed as fault-protection schemes; therefore, latency is not as important. They are often designed to repeat commands, so it is not critical if a message is lost or corrupted by fault-generated noise. Eventually, the message will get through. Control applications typically have an acknowledgement or confirmation that the message was received. If the message is lost, the control application sends it again. Attributes that are desired for both protection and control communications include the following: Reliability even in adverse weather conditions (when protection and control functions are typically needed the most). Security against operations caused by corrupt data from channel noise or even hacking by cyberintruders. 3
Common types of communications systems for protection and control functions include the following: Direct fiber-optic cable. Multiplexed fiber-optic cable (synchronous optical network [SONET]). Power-line carrier. Leased analog or digital phone line. Licensed radio or microwave. Unlicensed radio, such as spread spectrum. Others? The attributes of these communications systems are discussed on the following slides. 4
Direct fiber optics has many advantages, including the following: Immunity to electrical interference (fault-generated noise). Speed (latency less than 5 microseconds per kilometer). Initial costs are proportional to distance; installed costs range from about $10,000 per mile incremental cost (versus standard shield wire) for optical ground wire (OPGW) installation on a new transmission line to upwards of $70,000 per mile for shield wire replacement on an existing line. Low maintenance. There is no maintenance required on the optical fiber unless a fiber breaks. The optical transceivers may degrade over time, but they typically do not require any routine maintenance. Repeater needed for long distances. The transmitter optical output and receiver sensitivities typically limit distances to less than about 80 miles without repeaters. Exact distances depend on the optical budget and fiber and splice attenuation, with a margin to account for future repair splices and aging. The low latency, wide bandwidth, and immunity to interference make direct fiber a great communications medium choice for virtually all protection and control scheme applications. However, because the cost is proportional to distance, it is better suited to short-line applications. 5
Multiplexers can improve the bandwidth usage on high-bandwidth optical fibers, which allows more data to be exchanged over the same pair of fibers and more users and applications to share the same fiber pair. The multiplexing process adds a delay, which impacts the speed of protection schemes that rely on the communications scheme for high-speed tripping. The delay is typically small, around a few milliseconds or less. While adding equipment to an optical-fiber path increases complexity and decreases reliability (compared with dedicated point-to-point fiber), multiplexers can also improve reliability through the design of a self-healing network. This allows data to be passed around failed multiplexers or fiber, thereby increasing reliability. This is often a fair tradeoff for the increased cost and complexity. A time-division multiplexed network, typically referred to as SONET, provides the most deterministic and consistent time delay for data or message delivery. This is an important consideration for high-speed protection schemes. Multiplexed fiber optics have the same distance limitations exist as direct fiber. The transmitter optical output and receiver sensitivities typically limit distances to less than about 80 miles without repeaters. Exact distances depend on the optical budget and fiber and splice attenuation, with a margin to account for future repair splices and aging. 6
The power-line carrier operates by injecting low-frequency radio band (30 to 200 khz) signals into the power line, using it as the communications medium between line terminals. The significant advantage is that the utility owns the power line. The communications latency is low, and transmission times are typically less than 1 power system cycle. Signals applied are typically single or dual frequencies, so the channel is a relatively low bandwidth. High initial terminal equipment cost is required for transceivers, capacitance voltage transformers (CVTs) with line tuners, and line traps. This communications medium is definitely susceptible to fault-generated noise within the line protection zone. The communications medium, therefore, is ideal for directional comparison blocking (DCB) scheme logic where communication is not required for faults within the line protection zone. It also works well with directional comparison unblocking (DCUB) scheme logic where logic allows tripping within a short window of time (typically 150 milliseconds) after communication is lost. Moderate, specialized maintenance is required to tune the line-coupling equipment and wave traps to work with the specific frequencies applied to the line. Most power-line carrier communications schemes used for protection either turn a carrier frequency on or off or shift from one frequency to another to signal a protection blocking or unblocking signal. There are digital power-line carrier schemes that send digitized data over the power-line carrier channel, making them suitable for SCADA. These are not well suited for protection applications because the digitized message is corrupted during an in-zone fault. 7
Leased analog and digital phone circuits offer low initial costs. However, the monthly charges (sometimes hundreds of dollars per month) continue, making the life-cycle costs quite high. Analog phone lines were traditionally used for audio-tone teleprotection schemes using frequency-shift transceivers operating within the voice band. Analog circuits are being phased out by the phone companies in favor of digital communication that can carry more bandwidth over the same communications medium. Analog circuits may still be available to the end user, but the communications circuit is likely to be at least partially digital and probably on a switchable digital communications network. Communications path switching creates uncertainty about the latency, which is often a critical attribute in protection schemes. In addition, switching and other digital signal corruption involving high-order bits can cause pink noise on analog channels that can fool the security in audio-tone teleprotection equipment, even equipment using dual frequency shift techniques, resulting in inadvertent trips. Digital circuits emphasize throughput; latency and reliability are generally not a concern for the communications companies that lease circuits. Indications from customers are that phone company technical support required to resolve questions about communications circuit performance has diminished and sometimes may not be available at all. Questionable speed and reliability make leased circuits the least desirable method for protection applications. Leased circuits are still adequate for SCADA and control applications that are not sensitive to high-latency communication or momentary disruption during faults or digital network switching. 8
Licensed radio communications channels have protected frequencies that eliminate interference from other transmitted signals in the same vicinity. The higher frequencies typically associated with licensed radio systems (in the hundreds of megahertz to low gigahertz range) make this form of communication relatively immune to interference by fault-generated electrical noise or lightning. Initial terminal equipment costs can be relatively high due to costs for radios, multiplexers, antennas, structures, radio path studies, and license fees. Distances are limited to line of sight, so repeaters may be needed if structures, trees, and topography block the radio signal path. The higher frequencies typically associated with licensed microwave radio are susceptible to weather conditions (e.g., rain, fog, and temperature inversions). High wind can cause deflections of the antenna structure, which can adversely affect the received signal. Multipathing caused by reflections can also cause jitter and signal corruption. 9
Unlicensed radios, such as spread-spectrum radios, have the following attributes: Low-to-moderate initial costs. Radios and antennas are relatively inexpensive. Structure cost to mount antennas range from nearly nothing, if there are existing structures available, to relatively expensive, depending on the height and type of structure required. A radio path study may also be required. Typically, there are no recurring fees associated with unlicensed radios. High frequencies typically associated with unlicensed radios (e.g., 900 MHz) are immune to noise generated by power system faults and lightning. Line of sight is typically required for unlicensed radios. Range can be up to about 20 to 25 miles. A user may need repeaters for longer distances or if line of sight is blocked by structures, trees, or topography. Unlicensed radios are susceptible to interference from other unlicensed transmitters in the same band. Spread-spectrum radios use frequency-hopping techniques to avoid most momentary interference and to prevent prolonged or continuous interference. Antenna polarization and SEL Hop-Sync transmitting techniques can reduce interference from collocated radios. Unlicensed radios are generally limited to low-bandwidth applications that are suitable for automation and control applications and for communications-assisted (low-bandwidth) protection schemes on lower-voltage transmission systems. They are generally not suitable for higher-bandwidth communications-required protection schemes, such as line current differential. 10
Communications medium costs can include both initial equipment and installation costs, as well as ongoing monthly or annual costs. Some estimated costs are shown in the table on the slide, along with the relative communications bandwidth provided by the communications medium. Direct fiber terminal costs (patch panel and fiber terminations) are relatively low, and ongoing monthly or annual costs are low. The main cost is the initial cost of the fiber cable and installation. Multiplexed fiber incurs the same optical fiber and installation costs, plus the cost of the multiplexers and their installation. Power-line carrier communication requires at least one transceiver, line tuner, coupling device, and wave trap at each terminal. Coupling devices must be rated for the line voltage they are connected to. Wave traps must be rated for the line capacity they are connected to. Some installations require coupling devices and wave traps on more than one phase. These costs can vary widely, depending on line voltage, capacity, and length. 11
Leased circuits have low initial costs but recurring expenses that become significant over time. Licensed radio or microwave systems are generally applied where high bandwidth is required. The channels are generally multiplexed to allow more data and users over the same path. The cost of the multiplexers, radio equipment, wave guides, antennas, structures, and license fees can be expensive. Unlicensed radios are relatively inexpensive, but have limited range, relatively low bandwidth, and may not be practical in some cases, such as in urban areas with structures that block the signal path. When choosing the optimum communications medium, a user must take into account the initial and recurring costs and consider the requirements for each specific application and the needs of present and future bandwidth. 12
The figure on the slide shows a comparison of the communications systems cost per mile based on the various communications systems considered to protect a line that is 20 miles long. This length is optimum for an unlicensed radio system, provided that the relatively low-bandwidth radio is suitable for the selected protection scheme. This comparison favors the communications medium with low-terminal cost and little or no cross-country investment. For reference purposes, the following costs were used: Fiber = $300,000 Power-line carrier = $75,000 Leased communication = $20,000 (present value of 20-year annualized cost) Licensed radio = $30,000 Unlicensed radio = $6,000 13
The figure on the slide shows a comparison of the communications systems cost for a 20-mile line in terms of the cost of megabits-per-second bandwidth. This changes the perspective dramatically, making fiber-optic communication the lowest cost option of megabits per second and leased communication the highest cost option. For reference purposes, the media costs per megabits per second were calculated using the costs for a 20-mile link listed on the previous slide and the following bandwidths: Fiber = 2,500 megabits per second Power-line carrier = 0.3 megabits per second Leased communication = 0.064 megabits per second Licensed radio = 155 megabits per second Unlicensed radio = 0.064 megabits per second 14
Other possible communications systems are available. Satellite, Internet, Wi-Fi, Cloud, and broadband power-line carrier options present some interesting opportunities. Most of these options present problems for protection applications because of their latencies, nondeterministic delays, and asymmetrical delays. The paper, Integrating Remotely Located Substations Into SCADA Systems: A Case Study Using Commercially Available Satellite Internet Service Providers for SCADA Communications, chosen for preconference reading discusses using satellite communication for SCADA with a very remote site. 15
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