Getting in (Switch) Gear
Editor's note: In a world of heightened sensitivity within electrical environments, engineers have been observing some interesting changes in switchgear and automatic transfer switches. For example, the introduction of digital trip technology in circuit breakers now makes it possible to communicate feeder data that previously would have required multiple instrumentation packages.
For example, the introduction of digital trip technology in circuit breakers now makes it possible to communicate feeder data that previously would have required multiple instrumentation packages.
Another noted change deals with higher continuous-current and short-circuit current ratings for low-voltage switchgear. This has allowed for a substantial increase in the power levels that can be accommodated in a single lineup.
CONSULTING-SPECIFYING ENGINEER: With the need for greater electrical reliability and redundancy in operations ranging from plants to hospitals to data centers, how has the application of switchgear changed?
DOWNER: Switchgear has always been the backbone of industrial power distribution systems, and it's even more important nowadays as it has taken on the role of a central nervous system. Today's electrical systems feature more sophisticated relaying and control functions, which include the need for additional metering and communications capabilities, as well as integral remote control and data acquisition. In the past, switchgear was primarily a brute force system to isolate electrical system components in the event of failure.
DALEY: But with respect to electric service, the role of switchgear has not changed for facility electrical loads. For low-voltage service—less than 600 volts AC—it is still derived through one of three low-voltage substation configurations: single- or double-ended, network grids or spot networks. That being said, I must point out the average number and rating of switchgear have increased as a result of increased technology penetration in the average facility.
DOWNER: Switchgear, itself, however, is more advanced. For example, logic functions were previously implemented with hard-wired control relay logic that tended to limit the scope of control system functionality. Protective functions were previously implemented by single-function, electro-mechanical relays that lacked flexibility and sensitivity. But modern switchgear utilizes microprocessor-based relay systems that provide programmable control, increased functionality and higher sensitivity that allow implementation of complex control functions such as load shed, bus transfer schemes and zone interlocking ground-fault protection.
DALEY: I'd add that the requirement for larger alternate power sources has led to paralleling more and larger engine generators on a common bus. Switchgear and switchboards for these alternate power sources have driven the continuous current ratings of this equipment to much higher current levels—up to 10,000 amps. Consequently, at these higher current ratings, the bus must be capable of withstanding significant fault currents. Because of the multiplicity of paralleled sources, the withstand rating of the bus must also endure these higher currents for multiple cycles.
CSE: Mr. Downer spelled out some of the technological advancements that have taken place in recent years. What advancements do you see coming next?
ALLISON: Communications for control and monitoring of on-site power will provide transmission and distribution service providers with dispatchable distributed generation.
LOVORN: I'd say there's a lot of room for growth with solid-state components. Recently, there's been a greater introduction of this technology—more solid state trip units and controls, and to a lesser extent, more solid-state power contacts, such as fully solid-state motor starters. If manufacturers can solve the problem of the low interrupting duty, it would be possible to produce an all solid-state circuit breaker within five years.
In fact, solid-state overcurrent, short-circuit and ground-fault trip units are already available and, as in the solid-state motor starter, they can interrupt full-load current. One solution might be speeding up tripping times to permit the device to open just past a current zero at the instant that the overcurrent is sensed. The three phases would have to operate 120 degrees out of phase with each other, but the total clearing time would still be under one cycle. This would open up an entirely new perspective on switchgear construction. However, the key will be community acceptance.
DALEY: I think we may see the incorporation of digital trip units into medium-voltage breakers, obviating the need for separate protective relays. Digital trip units are presently available with a multiplicity of trip functions. And we've already seen the application of vacuum-break contact structures in medium-voltage breakers (600 volt AC &15 kV AC). This development, of course, has reduced the switchgear footprint. The introduction of digital trip units better serves the need for power system information and central review of operating data.
DOWNER: I think a coming advancement will involve magnetic actuators. Innovations in medium-voltage switchgear have led to circuit breaker designs with smaller dimensions and lighter weight. This has also reduced the energy required to operate the breaker. This reduction in energy requirements will lead to the use of operating mechanisms based on magnetic actuators, which are considerably simpler to design, have fewer parts and are more reliable. These actuators also have a faster operating time, which will allow for rapid transfer switching between sources in the event of loss of a source.
Another area of innovation may come with capacitors, which can improve power factor and voltage regulation leading to more efficient operation. However, when a circuit breaker closes on a capacitor, the capacitor initially presents very low impedance and a large current flows until the capacitor voltage stabilizes. These energization waveforms can have negative effects on a plant's electrical systems.
Advances in circuit-breaker operating mechanism control will allow for the development of synchronous or zero-crossing switching. This involves each circuit breaker pole being independently operated to synchronize the completion of contact travel with the current zero crossing in each phase. Closing each phase's pole at the current's zero crossing minimizes—and theoretically eliminates—the inrush current peaks and voltage waveform distortion that occur during energization of the capacitors.
CSE: As far as the nuts-and-bolts of equipment operation, what kinds of switchgear nuances should engineers be aware of?
ALLISON: Perhaps the biggest factor is anticipation of potential future applications in the ever-changing world of generation. Systems are available that offer the ability to enable, at little or no cost, operations that become available at a later date. An example would be a transfer switch that can be reconfigured from open to closed transition, and utilizes the point of paralleling to the normal source for import/export or base loading operation.
DOWNER: Specification of switchgear configuration should be based on system operating requirements, reliability, future expansion and ultimately cost. The provision for future expansion is frequently overlooked, so retrofit work is often a challenge. Allocating space for future switchgear needs is a noble goal that is often fraught with peril because there are no definitions in the industry standards. Specifiers use many different, undefined words and phrases such as space , future space , equipped space , fully equipped space or spare . However, the specifier should be more exacting with his language so that there is no misunderstanding between purchaser and manufacturer. Another overlooked switchgear specification factor involves the effect of solar radiation for equipment located outside a facility. Obviously, an outdoor switchgear lineup is going to be hotter than an equivalent lineup inside a building. The problem with current ANSI standards is that they are all based on the presumption that solar radiation is not significant. Therefore, the rating and testing of the equipment does not include these effects. ANSI/IEEE C37.24 provides site-specific direction for derating switchgear exposed to the sun. The switchgear specifier must not assume that just because his specification indicates its location, the manufacturer is going to apply the above standard, especially in a competitive bid situation.
CSE: What are other common specification errors?
DALEY: There are times when it is inappropriate to specify one feature or another. Most errors occur as a result of an oversight with the best of intentions in mind. Design firms have an outstanding history for reputable and cost-effective design. Manufacturers and professional organizations, such as the Institute of Electronics and Electrical Engineers and the Electrical Generating Society of America, support the dissemination of timely information discussing the evolution of the industry. Because these organizations are exposed to a broader base of diverse applications than the typical design firm, they can be valuable resources.
LOVORN: The three most common errors our firm has seen are inadequate fault duty; unnecessarily complex and expensive customized designs; and lack of selectivity or coordination between devices. Designs with low fault duty are typically produced when the engineer fails to calculate the fault duty for the service or any of the panels. Complex designs and the lack of selectivity, while they are totally different in results, many times have the same cause—the engineer not taking the time to design a simpler solution or one that is coordinated. All three problems really stem from increased pressures in producing larger and more complex projects in less and less time.
ALLISON: Having been involved with many projects that expand the amount of generation and distribution, as well as the operational capabilities of that generation through the switchgear modifications, I have definitely witnessed a number of areas that are frequently overlooked. Because load always grows—usually sooner than anticipated—planning for expansion is seldom regrettable.
Operational modes such as zero power transfer, import/export control and base loading often become advantageous and are desired after the initial installation. These capabilities are usually much more economical to include in the initial design as opposed to modifying or adding to existing switchgear and transfer switches. Considerations such as fault interrupting and withstand capacities are often costly to increase and may even require the installation of new equipment. Consequently, the costs of overdesign should be weighed against the possible future function and growth of the system.
DOWNER: In my opinion, the differences in industry standards for the different types of low-voltage circuit breakers must be understood in order to properly apply them. Low-voltage power circuit breakers are defined by ANSI/IEEE C37 standards. Molded- case circuit breakers and insulated-case circuit breakers (ICCB) are defined by NEMA AB standards.
Molded-case circuit breakers of frame size 400 amps and below are 80% breakers. That is, they are rated to carry only 80% of their rated trip current continuously. Operation above this level will cause overheating and degrade breaker insulation.
Specifiers need to be careful with molded case circuit breakers of frame size 600 amps and above because they are available rated as 80% or 100%. The 100% rated breaker usually has a price premium over the 80% breaker, so it's important to be thorough in reviewing bids or shop drawings to assure compliance. Low-voltage power circuit breakers and insulated-case breakers are capable of carrying 100% of their rating on a continuous basis.
Rick Downer, P.E., Lockwood Greene, Atlanta
Ken Lovorn, P.E., President, Lovorn Engineering, Pittsburgh
Rick Allison, Technical Engineering Manager, Enercon Engineering, Inc., East Peoria, Ill.
James M. Daley, P.E., Consultant, ASCO Power Technologies, Florham Park, N.J.
In Search of Superior Switchgear Specs
What factors should engineers take into consideration when determining whether to specify a closed- or open-transition switch, or a three-pole vs. four-pole transfer switch? How about insulating buswork?
Let's answer the easy question first—insulating bus. Switchgear and switchboard bus is insulated. If the question is about encapsulated bus, then there are some issues that need to be addressed.
In medium voltage switchgear the bus is commonly encapsulated. However, the continuous bus rating is 3 kA and below.
The issue is more complex for low-voltage systems. Some manufacturers have bus encapsulation features for continuous bus ratings of 3 kA and below. The most likely incentive for this accommodation is the demand for a smaller system footprint, thus reducing the available volume for larger through-air and over-surface clearances.
For grid service switchgear, bus encapsulation presents little or no problem. For alternate power service derived from multiple paralleled engine generators on a common bus, system bus ratings are frequently above 3 kA and reach 10 kA; they may reach even higher current ratings in the future.
Open vs. closed load transfer
The load transfer strategy should be the most reliable strategy for the application. The highest reliability transfer strategy is open transition. This strategy will show blinking lights on test transfer and re-transfer after an outage or test. For motor loads, in-phase transfer effectively mitigates the back EMF phenomena for the open-transition transfer strategy. For transfer of transformer primaries between power sources, the magnetizing inrush can be avoided by closed-transition transfer only. However, it's important to remember this consideration: On loss of power, the transformer is transferred in the open-transition mode, thus the system-protective functions must be set to ignore this phenomenon. In such cases, open transition is more suitable.
Closed-transition transfer accommodates two system constraints: 1) restoration to normal power and 2) rating critical load transfer switches in comparison to the rating of the engine generator(s).
In regard to the former, if closed-transition transfer is used, the critical loads are re-transferred without interruption. Since the critical load composition is typically less than half of the facility loss, restarting non-critical loads can be time consuming. Closed-transition transfer of the critical loads leaves staff free to restart the non-critical loads.
As an added thought, high-intensity discharge lighting, having long restrike times, is best served by closed-transition ties.
Regarding load ratings, when a critical load segment comprises 25% or less of the alternate power source bus rating, then short duration (&100 ms) closed-transition transfer is a strategy that can make system testing practically transparent to the operation of the facility.
Should the transfer switch be a three- or four-pole configuration?
The possible permutations of power system configuration produce a number of ways to treat the neutral conductor of AC systems. Confusion can creep in when trying to make the best design decision for any given power system that incorporates both a preferred and alternate power source.
The National Electrical Code provides specific requirements for the treatment of the neutral conductor and provisions for its grounding. The purpose of grounding the neutral conductor in a defined manner is to assure protection against inadvertent excitation of conductive surfaces of equipment, enclosures, cable conduits and raceways.
The choice is yours
Which is the best way to go? There is no definitive answer. Whether it's an open- vs. closed-transition transfer, three-pole vs. four- pole, or insulated (encapsulated) bus, all are driven by application requirements, and most importantly, sound engineering judgment.
System v. Hardware
In the world of power generation, one change that's beginning to surface is a switch to interoperability.
Minneapolis-based power equipment producer Cummins Power Generation is one manufacturer espousing this practice in making its equipment compatible with almost any generator or piece of switchgear, regardless of brand.
"We want it to be compatible with other switchgear technology," says Kirk Adams, a product manager in the company's switchgear and networks division. "In today's climate, you can't afford to be hardware specific. So instead, we're trying to standardize our architecture to get around this."
Having a uniform hardware platform, he explains, means the system, as a whole, should be more reliable. In fact, Adams says there really is a change of culture happening on the manufacturing side in that there is a switch from products being designed and marketed around hardware toward a more systematic and functional approach.
"So far, engineers we've spoken to are receptive to the idea, but it will still take education," says Adams.
That's the key, adds Gary Olson, Cummin's technical counsel. Unfortunately, he says there's a Catch 22 in place in that many users want interoperability, yet the lowest possible cost. "In such case, interface and interoperability are the first things to go."
Furthermore, the company has found that not everybody wants to play ball. "They [other vendors] will give us information how to connect it, but they never give us information as to how to parse it," says Olson.
Still, the company is moving forward with this systematic vs. hardware approach. "We feel people are really just looking for solutions," says Adams.
The bottom line, adds Olson, is "what do you want us to do?"
The next step, in his opinion, is the need to implement an interface standard for interoperability.