Medium Voltage for Maximum Effect
Medium-voltage power distribution systems are common in industrial settings and are becoming common in other types of facilities. However, each type of facility has its own special requirements.
CSE: Besides industrial plants, what other types of facilities rely on medium-voltage systems? How does specification of these systems differ in non-industrial settings?
GUSTIN: Actually, many building types require medium-voltage systems: health-care, office high-rises, sports complexes, data centers and waste and water treatment facilities. Installations can range from simple radial systems to loop systems to multiple-source primary systems. Consequently, system selection can vary dramatically. A few examples are:
College campuses using a primary loop configuration for ease of isolating cable faults.
Hospitals, with a great need for service continuity, requiring higher coordination in overcurrent tripping of circuits, higher integrity of medium-voltage equipment and various backup power schemes.
Waste and water treatment facilities requiring medium-voltage switchgear with the highest degree of reliability and maintainability. In fact, the addition of standby generation at the medium-voltage level is becoming more common.
TURNER: Yet another application is power plants, due to the large BOP (balance of plant) loads, such as boiler feed-water pumps and forced-draft fans.
The specification of medium-voltage in power plants doesn't significantly vary from industrial facilities. It has been our experience that power plant specifications can be more involved due to owner requirements, typically utility-based. Even independent power producers usually have specific requirements resulting from ties to utility requirements.
KAMINISKI: As for how specifying medium-voltage differs between industrial and these other types of non-industrial facilities, in the former, electrical loads may be so large that low-voltage distribution is not feasible. The actual utility service to the site may be at sub-transmission or transmission levels.
For non-industrial facilities, not only the size of load, but also the practices of the local electrical utility determine the selection of distribution-system voltage. Local utilities may have greater influence on the specification requirements for both the equipment construction and protective relays in a non-industrial setting. In other words, a medium-voltage service in one utility's service area may be 600-volt service in another utility's service area.
Another significant specification difference in non-industrial settings is the absence of a centralized engineering department. For example, a large industrial user may have "standardized" equipment specifications, but an institutional or other user may rely more on consulting engineering firms or equipment manufacturers for specification recommendations.
CSE: What are the latest technological advancements in these systems?
GARLAND/PAPADEMOS: Well, for one, compactness of switchgear makes it easier to integrate into non-industrial settings. Also, advancements in electronic metering systems and equipment provide ready access to system data needed for proper decision making from the owner's end. Finally, advancements in electronic/solid-state medium-voltage relays make coordination with both upstream and downstream devices easier.
GUSTIN: More specifically, some recent technological advancements include:
Switchgear designed for "corona-free" construction.
Arc-resistant switchgear that can deflect the blast and explosion of any internal switchgear fault to the sides or out the top of the switchgear.
Maintenance-free vacuum-switch switchgear for loop applications.
Front-accessible medium-voltage switchgear.
Medium-voltage drives and soft-start solid-state starters.
Medium-voltage transfer switches being used more and more for single-generator-set applications.
Constant interrupting current vacuum breakers, K=1.
Self-powered protective-trip units for vacuum breakers.
TURNER: As for the industrial users, they're requiring higher levels of safety for the operation and maintenance of equipment. Switchgear suppliers are responding by marketing the types of equipment that Mr. Gustin describes, such as arc-resistant switchgear and arc-protection relay systems.
KAMINISKI: I feel that the most visible change is the proliferation of electronic relays, metering and communications systems within the equipment. One multi-function relay basically has every relay-protection function available that a power system engineer may need. Equipment-mounted metering can now provide waveform capture, sag/swell detection and transient detection, as well as trending and forecasting functions. In the past, this type of functionality was limited to portable analyzer-type devices. The built-in communications capabilities of these devices now allow all user levels access to the same kind of power system information that was once reserved for utility engineers with dedicated SCADA systems.
Also, the type of equipment that Mr. Turner alludes to, such as active arc-extinguishing systems, uses current and optical sensors with a high-speed shorting switch to detect the presence of an arcing fault within an equipment enclosure, and extinguish the arc before pressure within the enclosure can build to a significant level. This technology is superior to the traditional "passive" arc-resistant enclosure construction that seeks only to direct the byproducts of the arcing-fault condition by venting. The traditional method does nothing to extinguish the arcing fault, and the equipment is subject to significant damage.
CSE: What common errors do you see in medium-voltage system design, and how are they avoided?
KAMINISKI: Specifying an isolated neutral bus, which is not required, in most cases, in medium-voltage distribution systems, is a common error. This is a carryover from 600-volt-class equipment specifications.
Another common error in medium-voltage design is failure to provide for a reliable source of control power for circuit breaker tripping and for microprocessor-based protective relays.
TURNER: One recurring mistake is in the specification of connection boxes for medium-voltage motors. The standard box provided may not allow for the proper termination of the medium-voltage shielded cables. As a result, many field-fabricated boxes come about as a result of this problem. Engineers should specify oversized connection boxes for these motors, or if owner specifications allow, non-shielded cables operating up to 2,400 volts should be provided.
GARLAND/PAPADEMOS: In addition to system performance under normal conditions, the behavior of the system under fault conditions needs to be analyzed and lightning arresters need to be specified accordingly.
With regards to relays, their selection must be made in close coordination with the utility. Existing utility relaying and settings must be taken into consideration when specifying the equipment for medium-voltage system protection. Also, utility upgrades scheduled for the near future must be carefully considered.
Medium-voltage motor starters should also be specified as part of the overall system, rather than purchased on the basis of "readily available" equipment.
Yet another important design point is the location of switchgear. Equipment associated with medium-voltage systems comes in size, appearance and maintenance requirements that are different from those used for low-voltage systems. Consequently, attention must be paid to the following considerations:
Switchgear should be positioned so that service vehicles have ready access for maintenance.
Ample space should be provided to easily accommodate required clearances, maintenance access and maneuverability for replacement or expansion.
Aesthetically, equipment should be as unobtrusive as possible.
GUSTIN: Another common error is applying medium-voltage vacuum circuit breakers at voltage ratings below their nominal ratings; a lesser short circuit MVA rating can often occur for breakers whose K rating is over 1. For example, a 15-kV, 500-MVA-rated vacuum breaker—where K factor is equal to 1.3—applied at 4.8 kV can no longer interrupt 500 MVA at that voltage. Essentially, the breaker cannot interrupt ever-increasing current at lower voltages to keep a constant MVA interrupting rating. Formulas for calculating this are available from most manufacturers.
Finally, it's not unusual to see projects specified with metal-enclosed, load-interrupter switchgear for this application. ANSI C37.20.3-4 determine duty-cycle ratings for this type of load interrupter. The ANSI standard number of load-interrupting operations for a 15-kV, 600-amp switch is 30 times. With any routine load testing or failures of a source requiring a transfer of load, this selection may not assure minimum longevity requirements.
Cost pressures are probably more intense today than ever before. Value engineering and alternative designs by bidders increase the need for designers to know what is gained or lost in the selection of medium-voltage switchgear. The differences in construction, size, cost, operation, protection and maintenance are dramatic.
CSE: What are some of the power protection considerations for medium-voltage systems?
TURNER: Medium-voltage systems often service a greater number of loads because they are distributing power to a larger part of the electrical system. This means that continuity of service becomes more of a concern than for the lower-voltage systems. Longer time delays may be set on relays to allow selective coordination, and high-speed bus transfers may be required.
GARLAND/PAPADEMOS: The capabilities of existing equipment to interrupt "new" available faults need to be carefully examined. And the ratings of new equipment must be such that they easily accept system modifications instituted by the utility or facility owner in the future.
Also, the ease and convenience of having protective devices transfer automatically to an alternate source in the case of a "normal" source failure needs to be closely analyzed. Complex operating conditions may be needed to achieve the scheme.
Finally, the use of vacuum breakers vs. SF6 breakers vs. fused devices needs to be closely examined with respect to cost, the ability to coordinate with the utility's existing and anticipated protective schemes, and the ability to properly protect existing downstream equipment.
GUSTIN: More special considerations include the following:
With increased concerns over utility reliability, many facilities are entertaining larger standby power systems. In many cases, it makes sense to input the standby system generation at medium-voltage levels. The tie-in therefore occurs somewhere in the "normal" medium-voltage system. This requires careful assessment of protection issues such as increased short-circuit currents; additional protective relaying such as differential protection; synchronous check relaying and more. It also mandates careful assessment of the sequence of operation of circuit breakers that are typically automated. Ground-fault sensing issues can become complex, owing to multiple-source grounding as well.
Power-monitoring systems (PMS) are also commonly specified for many facilities with medium-voltage distribution systems. The PMS can contain precursor information about potential fault problems, or the likelihood of such, to assist building operators in assessing equipment that may need investigating for repair or maintenance. With fewer facility operators, this can be an effective method of increasing building uptime or continuity of service. It is important to carefully specify the local area network that the design engineer intends for that system, how other communication devices may enter into the system and how the owner may access the data.
Whether a system has automatic throwover between multiple utility sources is quite important from several standpoints: If the return to the normal source is done in a closed-transition manner—i.e., power is not lost during this operation—circuit-interrupting devices may need to be rated for both sources' short-circuit contributions. Furthermore, if paralleled operation is being considered, appropriate relaying to guard against back feed to a utility source must typically be employed. Also, one needs to determine the type of interrupting devices that are specified for automatic throwover systems. Typically, at medium-voltage, these should be vacuum breakers.
CSE: One final question about protection: What can you tell us about the implementation of partial-discharge (PD) monitoring in medium-voltage systems?
GUSTIN: PD technology has evolved into a standard product offering for monitoring power transformers, medium-voltage motors and medium-voltage switchgear.
We feel that it's the first and single best online predictor of potential faults before they occur, at least in medium-voltage switchgear. This gives the specifier a method to design a system where an alert is sent before catastrophe strikes, namely a potentially faulted lineup of medium-voltage switchgear. Once it is faulted, it may be days before the owner can get back to normal operation, depending on the severity of the fault. Liability issues for the designer and owner in the event of injuries to operators are also a reality of today.
The system's basic operation is to employ sensors to detect the high-frequency signature of PD. Protective relays are connected to these sensors and interpret the signals to determine if corona is occurring. Protective relays can be mounted in separate cabinets or lineups of medium-voltage switchgear. Typically, an alarm signal is sent either by dry contact, pager or communications system to alert building personnel to the potential problem. This allows the owner to assess the risk of a possible upcoming insulation failure, schedule a shutdown at a time that minimizes disruption to the facility and take corrective measures to repair or replace or rewind.
GARLAND/PAPADEMOS: The best way to implement PD is to institute a formal program. There are a number of testing companies that can perform this type of testing on medium-voltage systems without power interruption. These companies will collect data, analyze the results and provide recommendations as to the action that should be taken and associated time frames. If one has reason to suspect that one's distribution system may be experiencing cable insulation problems, contact one of the vendors offering this service. For a new system, the same vendors will be able to recommend a preventive program.
KAMINISKI: However, I'd offer a word of caution here. PD monitoring-system manufacturers have been attempting to promote their systems in switchgear. While any continuous monitoring of equipment condition has value, the benefits of continuous PD monitoring in 5-kV to 15-kV switchgear applications may be difficult to justify and are not a substitute for proper equipment maintenance.
Phil Garland , P.E. , Sr. Electrical Engineer, Albert Kahn Assocs., Detroit
Bob Gustin , Fellow Application Engineer, Eaton Corp., Novi, Mich.
Paul Kaminski Staff Marketing Specialist, Schneider Electric, Nashville
Tom Papademos , P.E. , Director of Electrical Engineering, Albert Kahn Assocs., Detroit
Randy Turner , P.E. , Manager of Electrical Engineering, Lockwood Greene, Atlanta
Grounds for Debate
When it comes to grounding strategies, one can count on electrical system designers to offer a wide range of opinions on the topic.
"The most commonly used system [for medium voltage] is high-resistance grounding," says Phil Garland, P.E., with Albert Kahn Assocs., Detroit. "It is frequently used in association with the operation of large chillers, air compressors and other similar equipment."
Garland explains that it provides an acceptable means of keeping equipment operating during ground faults, which must be corrected at the earliest possible window of opportunity to avoid the possibility of subsequent catastrophic faults.
But others beg to differ. "We prefer a low-resistance ground for medium-voltage systems, a typical value being 400 amps," say Randy Turner with Lockwood Greene's Atlanta office. Turner feels that a high-resistance ground may not let enough energy through to allow relaying to pick up during an arcing fault, and a solid ground may allow too much energy release during a fault, thus damaging equipment and motors.
Bob Gustin, applications engineer with Eaton Corp., Novi, Mich., agrees with Turner about the type of system. "Yes, generally, the preferred grounding for all medium-voltage systems should be low-resistance grounding," he says, "where selective coordination can be achieved for ground faults." He feels that high-resistance grounding of medium-voltage systems is recommended only where continuity of service is paramount and facility operators have a planned method to quickly locate and remove any ground.
"There are textbooks, like the IEEE Red Book Std 141-1993, Chapter 7, dedicated to this subject," says Paul Kaminski with Schneider Electric's Nashville office. "But I'd have to say that in the majority of medium-voltage distribution systems, solidly-grounded or low-resistance grounding systems are used. The type of grounding system used generally depends on who is providing the service transformers." If the utility company is providing and maintaining the main service transformers, then the power system is usually solidly grounded, he says, as the utility prefers to have better control of its relays during faulted conditions.