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EQUIPMENT PROTECTION
Transmission, Distribution and Substation Equipment Protection
By A.J. (Tony) Surtees
With the move towards deregulation within the power utility industry, customers are demanding superior power quality and reliability of supply. Many utilities have responded to the needs of their customers by establishing power quality divisions within their marketing departments. Never before have the demands for continuity of service, performance-based rates and penalties for inconsistent reliability, been more apparent to utility companies.
One response to this demand has been to automate many distribution nodes to allow greater flexibility in controlling the overall supply network. Unfortunately, such flexibility has come at a cost -- the greater use of electronics in remotely automated systems has meant that vulnerability to over-voltage disturbances from lightning and switching transients has increased exponentially. The need for mitigation protection at such sites has become critical.
Lightning protection, grounding, equipotential bonding and surge protection are all interdependent disciplines within a total facility protection plan. Reliable protection of facilities and personnel demands a systematic and comprehensive approach to minimizing threats from lightning and power system disturbances. Electrical protection is especially critical where electronic communications and supervisory control systems are used, as these are particularly vulnerable to the effects of over voltage transients.
The following Six Point Plan has been developed as a checklist when considering the protection of facilities such as substations and switchyards. The concept behind the plan is that it prompts the user into considering a holistic approach to lightning protection, one embracing all aspects of potential damage. From the more obvious direct strike to the more subtle mechanisms of differential earth potential rises and voltage induction onto power and control cabling, the Six Point Plan is technically correct, cost effective and compatible with future equipment needs.
The six points of the plan are:
- Capture the lightning strike to a known and preferred attachment point.
- Safely convey this energy to ground.
- Dissipate the energy into a low impedance grounding system.
- Bond all ground points to eliminate ground loops and create an equipotential plane.
- Protect incoming AC power feeders.
- Protect low voltage data, signal and control circuits.
Capture The Lightning Strike to a Known and Preferred Attachment Point
This involves the use of an effective air terminal system to provide a protective area for the ground-based equipment. In a substation, this would include the transformers, auto-reclosers, dead ends, SCADA RTU cabinets and others.
In the design of area protection, it is important to realize that many points within the facility being protected will be competing for the downward lightning leader by launching upward intercepting streamers when the critical air breakdown threshold is reached. A measure of the effectiveness of a lightning protection system comprising strike termination devices (air terminals) is its ability to respond by launching interception streamers. These streamers will thermalize into propagating up leaders before any object(s) within the protected space itself launch competing streamers.
The earlier the streamer launches, with respect to extraneous emission points on the surrounding structure, the better the air termination system will be in ensuring it will take the strike (at a known point), thereby preventing random striking or bypasses to adjacent uncontrolled points. It is for this reason that alternative air terminals are being considered by many utilities for the protection of their substations and switchyards. Not only are these systems designed to become the preferred termination points in the event of a lightning leader approaching to within the striking distance of the site, but they also obviate the need for catenary shield wire systems. Such sky-wire systems are recognized by many utilities as a "necessary evil" over their facilities, which pose risks in themselves. These include the risk of fusing of the catenary wire in the event of a direct strike and having this wire fall across the live bus work. Another commonly cited problem is the nuisance factor posed by such overhead catenaries when needing to operate cranes within the switchyard.
Safely Convey This Energy to Ground
This involves the use of a dedicated, down conductor capable of withstanding the full energy of the lightning discharge, and conveying this to the grounding system, with minimal danger of side flashing to adjacent objects or metal work. All too often, the down conductor system is under-rated in the role it plays when conveying the discharge current to the ground. It is often over simplified to being little more than a copper or aluminum conductor of adequate cross sectional area to handle the current, which may be as high as 150 kA. A more theoretical study of the down conductor reveals that the characteristic impedance at the frequencies of the lightning transient can often be much higher than expected, compared to when simply considering the DC characteristics. Under transient condition, this dedicated path-to-ground may appear less attractive to the lightning when compared to some nearby alternative where a side-flash may then occur. Careful design of the down conductor, using elements of network theory related to high-voltage coaxial cable designs, can result in a better cable with very low self-inductance and characteristic impedance at the relevant frequencies of the lightning transient.
Dissipate The Energy Into a Low Impedance Grounding System
The need to understand the characteristics of the grounding system under impulse conditions (associated with the higher Fourier spectral components of the lightning discharge) is crucial if an effective grounding system from a lightning perspective is to be designed (and not just low-frequency fault current perspective). An effective grounding system is one in which the potential rise of the surrounding earth is minimized, the rate of potential fall off from the injection point is maximized and the high frequency components of the lightning discharge are safely arrested without substantial surface discharges occurring. Use of a "leaky transmission line" model can be very useful in such analysis.
Bond All Ground Points to Eliminate Ground Loops and Create an Equipotential Plane
Ensuring that a single point, grounding policy is adopted, and that equipotential bonding is used throughout the installation, will help eliminate differential ground potentials which are not only dangerous to personnel but also can cause damage to equipment. This includes the sensitive RTU controllers where multiple ports (often with different ground references) may terminate. As an example, a SCADA controller may have a telephone line and a power supply terminating on the PCB. If the ground references of each of these separate services are not electrically bonded together, a potentially destructive voltage gradient may be impressed across the sensitive electronics during a transient condition.
Connections are often the weak point of the electrical circuit -- especially circuits that are subjected to corrosion and high current. The capacity of the grounding circuit to protect property and personnel depends on the quality of its connections. Using exothermic connections is recommended. They provide a permanent molecular bond that cannot loosen or corrode. Properly installed exothermic connections carry more current than the conductor, resist repeated fault currents and can be quality controlled by visual inspection.
Protect Incoming AC Power Feeders
Generally, transmission, distribution and substation equipment is protected from lightning using distribution class surge arresters designed to limit voltage transients to the BIL rating of the transformers and lines. These measures provide excellent protection for the electrical network but are totally inadequate to protect sensitive electronic equipment such as the SCADA control system and substation data concentrator computers commonly found in modern automated facilities.
Effective protection of such electronics involves the installation of voltage-clamping devices on the AC feeders to power supplies units which, in turn, supply DC power to the circuit boards. This type of surge protective device must be capable of handling the large energy content of the over-voltage surge (kA rating), as well as reducing the extremely fast rising edge (dv/dt and di/dt) of this transient.
Protect Low Voltage Data, Signal and Control Circuits
A SCADA system responsible for the management of various pieces of equipment within an automated switchyard or distribution network may have a myriad of inputs. These generally include: remote RTU transducers; current transformers; potential transformers; limit switch contacts; alarm contacts; and the like. Interfacing the sensitive electronic cards and field-mounted transducers can appear like large antennas reaching to remote ends of the switchyard and collecting induced voltages from lightning surges, switching transients or system faults.
Effective protection of these control and signal lines usually involves the installation of high-speed protective transient barriers. For effective voltage limiting, a hybrid circuit is usually employed where both speed of activation and energy handling capacity, are optimized in a multi-stage protective module.
Conclusion
Deregulation of the supply of power has seen utilities move towards greater automation and centralized control of their transmission and distribution networks. This has necessitated the increased use of sensitive electronic interfaces. Implicit in this is a greater susceptibility to damage from over-voltage transients created by lightning and other system operations. The need to adequately mitigate against such damage has required more attention to electrical protection than in the past. The Six Point Plan approach to such protection ensures that all elements of a facility's vulnerability are addressed in a coordinated and effective way.
Tony Surtees holds a BSc (Eng) and PhD from the University of Cape Town, South Africa as well as an MBA from the University of Tasmania. He is a recognized expert in lightning and surge protection and currently holds the posts of Director, Standards and Technology and Product Manager, Surge Protection with Erico Inc. in Cleveland Ohio. He is a US delegate to the IEC Technical Committee TC81 on Lightning Protect-ion and SC37A on Surge Protection. ET
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