By Bob Fesmire, ABB Inc.
In the beginning, there was the electromechanical relay. With the advance of technology, solid state and later microprocessor relays began to be installed in substations, but while they offered tremendous improvements over their predecessors, they also came with at least one major drawback. Modern relays, also called intelligent electronic devices (IEDs), don’t require regular testing and calibration, and today’s IEDs can do in the space of a bread box what it once took a series of eight 8-foot-by-3-foot panels to do. Their Achilles heel, though, is a susceptibility to electromagnetic interference (EMI).
Equipment like the managed field switch pictured here are engineered specifically as “hardened” substation devices.
Overcoming this vulnerability was perhaps the genesis of the present-day concept of “substation hardening,” though that term is now often used to refer to protection against cyber attack. The recent obsession with cyber security is well-founded-the threat is real, and the utility industry’s reliance on “security by obscurity” no longer holds, as arcane proprietary systems are replaced with widely used commercial products and open standards. However, the likelihood of cyber attack is relatively low in comparison to disruptions caused by natural forces like thunderstorms or switching transients caused by operational activities. For this reason, it is most useful to consider the concept of substation “hardening” in the broadest sense.
Hardening a substation should imply making it less vulnerable to the full range of threats to equipment integrity and system reliability, be they natural or man-made. These include switching transients, as noted above, as well as EMI from lightning strikes, electrostatic discharges, and even interference from work crew radios. Environmental conditions such as extreme temperatures and garden- variety mechanical failures in equipment like hard drives round out the threat list. Hardening also means reducing the emission of potentially disturbing surges or interfering noise that could harm other devices inside or outside the substation.
Combating all these forces requires a combination of hardened products and robust system design. On the product side, IEDs and the communications hardware that supports them must be able to withstand the rigors of the substation environment. RTUs with closed panels and no moving parts, for example, are preferable. In terms of design, redundant architectures that make use of standby servers or embedded gateways in parallel to a software communication server can be used to bolster the availability and durability of the substation automation scheme. The design of the communications network topology is also very important, though as with the other elements noted here, there is a cost-performance tradeoff to be made.
Hardened substation automation systems manage or eliminate threats in a variety of ways. As mentioned above, IEDs have gone to using flash memory in place of hard drives to eliminate the potential for failure associated with moving parts. RF interference created by the noise of surrounding devices is mitigated by the use of fiber optic cable instead of copper wire.
In the case of dealing with switching transients-perhaps the most problematic threat to IED performance-there has been an interesting evolution in product design. Early solid state relays had problems with maintaining operations through switching transients that would come through the substation battery, ironically with the potential to cause the device to fail at the very moment it was needed most. So, relays started to come with their own power source in the form of a NiCad battery, but constant recharging was a problem due to the battery’s “memory.” Eventually, power supplies improved and today’s IEDs meet transmission-grade surge specs, enabling them to make use of the substation battery once again.
Substation communications equipment has also come a long way. An engineering adage states that communications equipment should be as resilient as the IEDs it serves. Ideally, such devices should not only continue to operate during a worst-case scenario, but the quality of the data they transmit should not be degraded either. Replacing copper wire with fiber will make data flows impervious to EMI, but system design is also a tremendous influence on resiliency.
There are two basic communication architectures, ring and star, with many variations and hybrids based on each. Ring architecture links a series of switches in series in a circle, thus providing each switch with two pathways (via the other switches in the ring) back to the starting point, or root. Star architecture links switches directly to the root independent of one another.
Ring designs offer higher availability and redundancy, but they cost more in cabling and are also slower since data from a given switch must pass through others to get back to the root. There is also the problem of “loops” where data continues to circulate around the ring again and again. To overcome this, ring architectures rely on so-called managed switches that have intelligence built into them that prevents loops from occurring. Due to the reliability of Ethernet switches compared with bay IEDs, the ring is recommended in general, but star architectures offer greater speed and lower cost and are often used within an enclosure.
This is, obviously, a simplified analysis, but it serves to highlight the essential tradeoff in benefits between these two designs. Hybrids that use, for example, a ring made up of smaller star configurations seek to offset the drawbacks of one with the benefits of the other. Physical distance between switches, the EMI environment and availability requirements will all have an impact on communication system costs, and for large systems a worst-case response time and throughput analysis should be done to achieve an economical balance between speed and redundancy.
Selection and screening of components and the choice of architecture are critical to any substation hardening initiative, but the cost/performance tradeoff will be unique for any given facility.
Bob Fesmire is a communications manager in ABB’s power technologies division, and writes regularly on transmission and distribution, IT systems and other industry topics. The opinions expressed here are his own and do not necessarily represent those of ABB.