Switching Power

08/01/2006


Innovations in controls and communication systems have improved monitoring of switchgear and transfer switches. Manufacturers are also touting arc-flash-resistant designs.

CSE: What are the latest design innovations in switchgear and automatic transfer switches (ATS)?

LIGGIO : In brief, equipment is smaller, communications have improved and arc-flash safety has increased.

OLSON : There are new arc-resistant and arc-proof designs, but in general, these new capabilities are not yet hitting a large number of projects. The biggest advances are probably in the area of controls—both the integration and advancement of common feature sets and the advent of easy, low-cost communications, as Mr. Liggio mentioned. This is changing end-user expectations of standard system features, especially when provided with standby power.

For example, “ramping” closed-transition transfer of sources is becoming pretty much standard on larger projects, and many more projects have both touch-screen operator panels and web interfaces to equipment.

MEULEMAN : We've been seeing a growing interest by end users in system simulators. Such simulators duplicate the control system of the on-site power system and are used for both training operating personnel and testing various failure modes.

Because these on-site power generation systems are typically found in mission-critical applications, hands-on training with the actual equipment is very difficult to schedule. Testing the failure modes for system response is even more difficult to schedule with the actual equipment. Using a simulator does away with the scheduling conflicts. If a mistake is made during training, or if a circuit breaker is signaled to open during a failure scenario test, no real load is affected.

Some of these on-site power systems can be quite complex. Simulators give operating personnel much greater familiarity and confidence with the equipment before “going live.”

There is also a growing trend toward monitoring power in these systems to report conditions into an overall building power management or SCADA system. Frequently, that power monitoring is at the output of the transfer switch or transfer equipment so that power delivered by both the normal source and the on-site power source can be monitored at a common point. The monitoring can range from basic values to sophisticated waveform capture, trending and harmonics analysis. The data can be used to observe and adjust for load growth; detect differences in power supplied by normal source and on-site power source; and uncover power quality problems.

Furthermore, there is a growing trend toward event-recording. This type of data is vital to analyzing a system disturbance and determining its cause. Time-stamping of these events is critical to that analysis, and some systems have gone to the extent of synchronizing time via satellite. Selecting and coordinating the points to be monitored and the protocols and communication methods for this information is a significantly growing portion of the equipment design and manufacturing effort.

CSE: What must one consider in specifying appropriate circuit breakers for switchgear?

LIGGIO : It's important to take into account nominal voltage, interrupting capability, closing speed, discrimination with upstream/downstream circuit breakers, electric vs. manual closing, and the environment—and whether communication is required.

LOVORN : I'd elaborate by saying that interrupting duty is the number one consideration, whether series-rated or not. If the breaker can't interrupt a fault, then it can't be used.

Second would be whether the gear is draw-out or fixed construction. Switchgear traditionally is draw-out only, but some people use the terms “switchgear” and “switchboard” interchangeably. If we're using “switchgear” as generic for either, then breakers could be either group-mounted and fixed or individually mounted. With fixed construction, maintenance on the equipment is much more difficult without a shutdown.

Finally, the tripping characteristics of the breaker must be selected so that it will coordinate with the upstream and downstream overcurrent devices.

MEULEMAN : As for fault-interrupting capability, it's important to determine for low-voltage class (600-volt) breakers. For example, do the trip units coordinate with the upstream and downstream protective devices? Also, should the breaker be manually or electrically operated?

OLSON : Breaker technology has come a long way in the past decade. We are now seeing features offered in molded case breakers (MCBs) that were previously only available in power breakers. Consequently, microprocessor-based MCBs with five-cycle motor operators can be used—and also misapplied—in lieu of power breakers.

So, it's even more important that engineers pay attention to core requirements: proper selection of voltage and current ratings, breaker fault levels, withstand ratings, I2T, equipment BIL ratings, etc.

Moreover, advances in microprocessor-based trip units aid in the selective coordination that is now required in many applications.

CSE: What are the advantages and disadvantages of closed-transition transfer switches?

TAYLOR : The advantages are seamless transition, without interrupting power when two good sources are available, and generator testing under load without power interruption.

One disadvantage is that it may require use of protective relays to prevent back-feeding power to the utility, which can add significant cost, especially in the soft-load type. Also, in a standard ATS, it does not prevent the block loading of the generator on a test, so even though there is no interruption of power, there may still be a significant sag in voltage or frequency.

LOVORN : I agree that a major advantage is transferring between two sources without an interruption. However, this advantage is greatly outweighed by the disadvantage of allowing the two sources to be connected together, even for the few cycles for the closed transition. This connection provides a major reliability risk for continuity of power to the load.

OLSON : It's also important to note that when switching to a generator set that has limited load pickup capability, there is always a disturbance, and this disturbance can be disruptive to system operation. Careful overall system design—mostly around sequence of operation and genset sizing—is necessary to achieve the desired result in the final application of the product.

Another major risk of closed-transition transfer is that if synchronizing is not done carefully, serious damage can be caused to the genset in the system. This damage is cumulative and can result in unexpected failure when the genset is needed most—during a real power failure.

Finally, closed-transition devices often don't have good failure-mode effects analysis in their design. A good strategy for fail-to-open and fail-to-close should be implemented to avoid equipment damage or danger to personnel.

MEULEMAN : There are actually two types of closed-transition transfer switch: the momentary-overlap type—typically 100 ms or less—and the soft-loading type. Both offer the advantage of not interrupting the load when transferring between live sources, such as when doing a system test or when retransferring to the normal source after an actual normal-source power interruption. For the momentary overlap type, the advantage is that it is a relatively low-cost means of providing this capability, only requiring the extra cost of both open- and closed-transition operation capability. However, one disadvantage is that care must be taken on sizing of the load being transferred. If the load is large in relation to the generator set rating, the fast transfer—100 ms—can block load the generator set, causing a voltage and frequency sag that may be detrimental to the load.

If multiple closed-transition switches are employed, care must be taken in sequencing them onto the generator to avoid the block-loading possibility. The soft-loading type has the advantage of smoothly and gradually transferring the power between sources, avoiding the block-loading possibility. But it has the disadvantage of additional cost, because controls must be added to ramp the engine governing system either up or down, depending on which direction the transfer is being made—not to mention the cost of connecting those controls to the generator. Because the transfer exceeds 100 ms, many utilities require protection of their system from the generator set, which they now see as a potential long-term power source that could jeopardize their equipment and personnel.This could result in adding protective equipment, with its additional cost, to separate the two sources if power flow back toward the normal source is detected. If multiple soft-load transfer switches are used, again, they must be sequenced as only one at a time can be controlling the engine governor.

CSE: How popular has integrated metering equipment, provided as part of switchgear or large switchboards, become? Are you seeing metering on just the mains, or is it common on the feeder devices as well?

MEULEMAN : Integrated metering is becoming quite popular. We are seeing it on the mains, but increasingly frequently on the feeders as well.

TAYLOR : Integrated metering equipment does seem to be fairly popular. The application for meters on feeders is significant, particularly on mains, but also where there are tenants and power cost is billed based on individual usage. High-end trip units that can provide metering and communication, in addition to basic protective features, continue to increase in popularity and are specified more often than non-metering trip units.

LOVORN : We have more and more owners asking for metering, both on the mains and feeder devices. However, we recommend devices that have unit costs of $250 to $300 per point, as opposed to the integrated metering that has a cost of $2,500 to $3,000 per point. We simply cannot justify the additional cost for the minimal increase in information.

OLSON: We still see plenty of open-transition standby/emergency systems, but many end users are opting for a more integrated generator/utility system. This means that the generator system is being integrated into the main plant or building's normal electrical service equipment. So now we often have to deal with various metering scenarios that allow end users to comprehensively meter the system.

CSE: In general, what is driving this metering trend?

LIGGIO : Basically, it's the minimal cost with increased information provided for the user.

LOVORN : The driving force is the desire on the part of the owner for real-time information for allocation of utility dollars to individual departments or functions. Some owners believe that metering will provide electrical energy savings, but we haven't seen appreciable reductions.

TAYLOR : Other drivers include an opportunity to provide tenant billing and address power quality issues, including sags, swells, transients, etc. Also, high expectations of the engineering staff, who are responsible for continuity of service on the electrical system, is a good reason to have metering in place.

CSE: Are you finding that end users are looking for fully insulated/isolated devices and buswork, such as was used in switchgear in years past, or is the trend, as a matter of cost reduction, more toward grouped devices with no bus insulation?

OLSON : It depends on the customer and the industry. The big issues here, of course, are safety, accessibility and price. I also think that it's important to look at what the electrical industry is doing. For example, if an insulated/isolated bus is specified on a medium-voltage system, then that is pretty standard, and everyone supplies that product. However, if one is working with a requirement for a simple low-voltage board for an emergency system, then expect to see longer lead times and added costs for fully insulated/isolated equipment designs.

LIGGIO : I believe the trend is toward fully insulated/isolated switching devices and buswork to minimize arc flash hazards and increase safety on working on energized equipment.

MEULEMAN : The concern for arc-flash protection is an argument for these devices. However, concern for footprint and cost reduction also remains strong. We haven't seen a strong turn in either direction.

LOVORN : While we recommend fully insulated and isolated equipment for maximum reliability, many owners are moreinitial-cost-driven, and the additional expense is more than they are willing to pay. We will continue to recommend fully insulated and isolated breakers and bus to optimize reliability since it prevents many accidental faults and, in the event of a fault, reduces the spread of the fault to the remainder of the switchgear. We have noted that the device isolation in vertical sections is not as tight as we would like, since we have seen fault propagation and spread of ionization products vertically in a section. If the sections were more tightly constructed, this spread would be minimized.

TAYLOR : Specifiers and clients who understand the cost/benefit situation are still requiring switchgear for their continuous processes and critical electrical loads. If the evaluation is being done with only initial cost as a consideration, we are seeing end users make the mistake of not buying switchgear for their critical loads and processes. Requests for insulation on equipment for environmental protection, where it is not a design or industry-standard requirement, continue to occur on an installation-by-installation basis.



Participants

Steve Liggio, P.E.
Engineering Manager
Point 8 Power
New Orleans

Ken Lovorn, P.E.
President
Lovorn Engineering
Pittsburgh

John Meuleman
Vice President
Russelectric
Hingham, Mass.

Gary L. Olson
Director
Cummins
Power Generation
Minneapolis

Paul Taylor
Product Manager
Eaton Corporation
Arden, N.C.

 

A Short Fuse?

While electronic fuses are certainly useful in a variety of electrical designs, how common are they for medium-voltage distribution design?

“While the concept of electronic fuses is great, we have seen little demand for their use,” explains Ken Lovorn, P.E., president and owner, Lovorn Engineering Assocs., Pittsburgh. “Virtually all of our medium-voltage applications use expulsion-type fuses.”

But others, such as Gary Olson, director of power systems design development for Cummins Power Generation, Minneapolis, say that they commonly see fuses for distribution-type equipment, but not for emergency systems. The main reason? “I believe end users prefer the reset capabilities of breakers over fuses, if possible,” says Olson.

At the same time, Paul Taylor, product line manager for automatic transfer switches, Eaton Corporation, Arden, N.C., seems to fall somewhere in between. He indicates that overall he's seen a number of specifications for electronic fuses—but only a minimal number.

“With only one switch manufacturer commonly using electronic fuses to date, we have not experienced their use in the field in volume. For the cost of electronic fuses, circuit breakers with self-powered trip units provide equivalent and added features as an attractive alternative,” he observes.



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