Transients in mission critical facilities

Each of the three transient conditions in an MCF has a different cause, different result, and different method of mitigation.

01/26/2011


The dictionary definition of a transient is “a momentary variation in current, voltage, or frequency.” While most discussions of transients for mission critical facilities (MCF) refer to short-term overvoltages and, to a lesser extent, short-term undervoltages, transient phenomena may be of any electrical unit, as noted in the definition. Each of these transient conditions has a different cause, different result, and different method of mitigation. The following discussion will investigate each of these and the kinds of effects that they have on the operation of an MCF.

Transient causes

Frequency transients are generally rare when the power source is the local utility, but they can be a problem when the utility fails and the emergency power system becomes the power source. Because the utility is normally connected to a power grid that has multiple generators and multiple systems—all interconnected—frequency variations cannot easily occur, so we will not concern ourselves with this scenario. Since most MCFs have emergency generators and uninterruptible power systems (UPS) as a backup to the utility, frequency stability is dependent on the control oscillator of the UPS inverter. Some UPS systems have multiple oscillators and they vote, with the majority of the controllers determining the system frequency and the minority controller is disabled.

Current transients are, normally, momentary current increases caused by a motor starting or short-circuit condition. As long as the current in-rush stays below the instantaneous/short time tripping curves of the fuse or breaker, there should be no effect on the system operation.

The most common transient conditions are voltage spikes and dips, which are short-term increases and decreases in the voltage of the distribution system. Most spikes, or increases, are caused by:

  • Switching surges
  • Conductor contact with a higher voltage system
  • Resonate effects of a series inductive-capacitive circuit
  • Repetitive, intermittent short circuits
  • Forced-current zero interruptions
  • Autotransformer connections
  • Lightning strikes
  • Harmonic voltages caused by the rapid switching of silicon-controlled rectifiers (SCR) in the rectifier sections of UPS and variable frequency drives (VFD).

Dips or decreases can be caused by motor starting, transformer energizations, large step loads, and resistance welding. It should be noted that the most frequent voltage spikes (switching surges in the utility distribution system, conductor contact with a higher voltage system, and lightning strikes) originate external to the distribution system, while the dips are normally caused by internal sources. Spikes caused by resonant conditions, intermittent short circuits, forced current zeros, and autotransformers are less common, but should still be considered because they can all cause significant damage to the distribution system or the connected equipment.

One common cause of high-voltage transients was the result of the opening of medium-voltage (1000 to 35,000 V) vacuum breakers, vacuum contactors, and current-limiting fuses. Manufacturers have made major design changes in the way the vacuum contacts operate so as to limit the effects of chopping the voltage wave and, therefore, limit the transient magnitude and frequency when they open. However, certain operating conditions and load characteristics can still result in damaging voltage transient conditions.

The high-voltage transient with which most people are familiar is the lightning strike. Lightning can be in the millions of volts and, upon striking an overhead distribution conductor, can elevate the voltage from a nominal distribution voltage of 12,000 V to over 75,000 V, or more than six times the norm if the lightning arrestor works as it was designed. If a lightning arrestor is not close to the point of the lightning strike or is not functional, the voltage on the system could exceed 500,000 V. Assuming that the lightning arrestor is working and the secondary of the transformer is 277/480 V, the voltage at the main distribution panel could be on the order of 1700 V, as long as the transformer did not fail due to the high-voltage transient on the primary.

How do transients affect MCFs?

Transients can have many deleterious effects on MCF operation and, without mitigation, can cause significant distribution damage, unplanned power outages, and loss of sensitive electronic equipment. Frequency transients in UPS output frequencies below 58 or above 62 Hertz will cause the UPS system to go offline and connect the load through the static bypass switch, directly to the generator, if the utility source is not available. This removes the isolation and battery backup by the UPS system, leaving the facility open to other transient conditions that might occur in the utility power system or in the emergency power system, when the utility power has failed.

Current transients will rarely cause problems for the power distribution system or damage sensitive equipment, but when the transient overcurrent persists for a period of time, the breaker or fuse could open and shut down power to the affected circuit. The result is an unplanned power outage on the circuit so that all of the downstream equipment will be out of service until the condition is corrected and the breaker reclosed or the fuse replaced.

Voltage dips will affect MCFs to varying degrees, according to the magnitude and duration of the individual dips. Short-term voltage dips (under one cycle) will normally have little effect on system operation because much of the sensitive electronic equipment can ride through them. Likewise, low-magnitude dips (less than a 20% voltage change) have little effect, so long as their duration does not exceed a second or so. Since one cannot tell the voltage dips to be of low magnitude or duration, the dips outside of these very restrictive parameters can cause equipment damage and outages, and must be addressed. Servers, PCs, and other nonmechanical equipment will simply shut down when the voltage drops too low for too long a period of time. For disk drives, damage to the magnetic surfaces can occur when the drive slows down without the read/write head being withdrawn, since they depend on a cushion of air to keep them from touching the surface.

Spikes, or short-term, high-voltage transients, can be more problematic for MCFs, since they can affect both the distribution and utilization equipment and the resultant damage can be a total loss of the equipment.

Avoiding transients

The easiest way to mitigate the effects of transients is to avoid them completely. (Yes, that seems to be obvious, but it is easier to say than to do.) Frequency and current transients are inherent in the power source or distribution system and avoidance is somewhat problematic, so we will address only their mitigation. Thus, avoiding voltage dips and spikes may be accomplished in several ways, according to the source of the transient.

The simplest to avoid are the voltage dips by carefully designing the distribution system so that the potential sources of a momentary voltage drop are isolated as far as possible, electrically, from the sensitive equipment in the MCF. Many MCFs will have a dual utility electrical service for redundancy, so motors that are necessary to the system operation may be connected to one service while the sensitive equipment is connected to the other. Obviously, during an emergency condition, when one service has failed, all of the loads will be connected to the same service, so care should be taken on starting motors that can cause damaging voltage dips during this time. Similarly, large step loads (and more rarely, any welding equipment) should also be connected to the second service for the same reason.

Avoidance of voltage spikes is possible for some sources and more difficult or impossible for others, which will require mitigation. The externally generated sources of voltage spikes, lightning strikes, and conductors contacting higher voltage systems are typically seen on overhead distribution systems, so having an underground, utility distribution circuit will eliminate many of these possibilities. If the utility has an underground distribution circuit from which the MCF may receive its power, the solution is simply to switch over to the less vulnerable circuit. It should be noted that underground circuits still require surge suppressors because there is still a possibility of switching surges and remote lightning strikes that can generate voltage spikes on the incoming service conductors. If a surge suppressor is not available, paying to have the overhead circuit installed underground will be prohibitive and virtually any mitigation solution will be more cost effective.

Design solutions to address autotransformer connections, current-limiting fuses, and resonate capacitors are the best way to avoid these potential voltage spikes. By using a full-voltage transformer instead of an autotransformer, i.e., a 277-240 V dry transformer instead of a 32 V autotransformer, a high-voltage spike may be avoided completely. Medium-voltage distribution systems may use current-limiting fuses or the slower speed, expulsion-type fuses for protection of the distribution equipment. In most of these systems, current-limiting fuses are not normally required and do not coordinate with the downstream overcurrent protective equipment. Additionally, the current limiting fuses, due to their very steep, time-current curve, do not fully protect the ANSI transformer damage curve, while the standard or slow-speed expulsion fuses can be selected to fully protect all points on the curve. For power factor correction, calculating the resonant point of the distribution system and sizing the capacitors to miss the resonance points will avoid this source of voltage spikes.

Repetitive short circuits cannot be avoided, so mitigation measures must be employed to prevent damage due to this transient source.

Mitigating transients

Transient mitigation techniques are primarily effective on voltage dips and spikes. Short-term voltage dips are handled by capacitive power supplies in many of the personal computers, servers, and printers, but the longer duration dips must have some type of power conditioner or uninterruptible power supply to maintain power to the MCF equipment. Power conditioners use large capacitor banks to boost the voltage during voltage dips and can handle magnitudes of up to 25% voltage drop (most units can maintain their output voltage within acceptable limits if the input voltage does not drop more than 10%).

Voltage spikes are somewhat easier to mitigate because there are multiple devices that can reduce or eliminate the high voltage conditions. Voltage snubbers are used for low-magnitude voltage control such as those generated by switching surges and wave chopping by vacuum breakers or current-limiting fuses. However, they do not work as well for high-energy voltage spikes such as those caused by lightning strikes, which are better handled by surge suppressors or lightning arrestors.

The best line of defense for voltage spikes is the surge suppressor. Surge suppressors are sacrificial elements where each of the metal oxide varistor (MOV), or similar suppressor elements, self-destruct to protect the downstream loads. For example, a small surge may be voltage limited by the suppressor without any MOV failures or one or two may be destroyed. For a large surge, many of the MOVs will fail to dissipate the energy of the voltage spike. Therefore, maintenance personnel should monitor the status of the suppressor modules, because a number of small surges can destroy several of the individual MOVs and the system cannot operate at full capacity when a large surge occurs. Surge suppressors limit the voltage to a peak value of about 1000 V as long as the surge does not exceed the capacity of the system. Performance may be improved by installing a large high-capacity surge suppressor at the service entrance, a less robust suppressor on the distribution panel, and a smaller unit at the branch circuit panel. The cascaded system will limit the voltage to considerably less than 1000 V, which is required to limit damage to the mission critical equipment.

Lightning arrestors installed on the main distribution panel can provide the initial voltage limitation at the service so that the surge suppressors do not have to work so hard to limit the voltage. Even with the use of arrestors, there should be multiple stages of surge suppression to reduce the peak voltage to a level that would not damage the MCF equipment.

It should be noted that in a typical facility, approximately 75% of power quality issues (which include transients, harmonics, and other disturbances) are created by internal sources. Therefore, having surge suppressors and lightning arrestors on the incoming service equipment will address only the disturbances from outside the facility, leaving most of the transients unmitigated.

Indirect transient mitigation

Sometimes a design will have a tendency to mitigate some of the transients, simply as a by-product of an alternative purpose. When power factor correction capacitors are installed due to a facility having a low power factor, the distribution system is impacted in several ways. Obviously, raising the facility power factor is one direct result and, if resonance and harmonics are not taken into consideration, nonpassive failure of the capacitors would be another direct result. Addition of capacitors has some indirect results, such as the elevation of the system voltage and, more germane to our subject, reduction of transient magnitudes. Due to the effect of a capacitor on steeply sided, voltage waves, the sharp rise time of a voltage transient is rounded off and somewhat reduced as the wave tries to charge the capacitor. Likewise, the trailing edge of the transient wave is stretched out as the capacitor discharges.

Like so many issues in electrical distribution, transient mitigation is very low on the priority list due to its infrequent occurrence. Therefore, facilities are reluctant to spend their limited infrastructure budgets on eliminating transients that rarely occur and whose damage is, in general, very minor. However, failure to prepare for the worst-case situation can be a really big mistake in MCFs. When one of those rare, large-magnitude transients occurs within or just outside an MCF, the results can be the loss of hundreds of thousands of dollars in critical and electrical distribution equipment. Due to this equipment loss, critical functions will cease and the loss of business, data, or your customer base, could drive the losses into the millions of dollars.

The moral of this story is to address these rare and unusual transient conditions before they happen. Yes, the money may have been spent somewhere else for a more immediate benefit, but failure to mitigate the high-magnitude transients could put you out of business.


Lovorn is president of Lovorn Engineering Assocs. He has more than 30 years of progressive design and engineering management experience with architect-engineers and consulting engineers designing electrical systems. Lovorn is a member of the Consulting-Specifying Engineer editorial advisory board.


Indirect transient mitigation by power factor capacitors

 

Indirect transient mitigationThere have been instances where power factor correction capacitors have reduced transients due to SCR switching in a UPS system to a level that did not affect any other equipment on the distribution system. This was confirmed when the capacitors were taken offline for maintenance and every printer in the building was taken out of service by the harmonics in the system. Once the capacitors were placed back online, the printers immediately began working with no adverse effects. Fortunately, the capacitors were not at a resonant frequency, or their positive effect would have been very negative since the resonance could have damaged the capacitors or caused them to have a nonpassive failure.


Critical operations power systems

The NEC has come out with some standards outlining what an engineer should do in MCFs to maximize power availability to the critical systems. While virtually all of the items are either covered in other sections of the code or mandated by good design practices for facilities of this type, this article gives the engineer a single-point reference for both. Of particular interest to the subject of transients in MCFs, NEC 708.20D says, “Surge protection devices shall be provided at all facility distribution voltage levels.” Therefore, surge protection, which has always been a good idea for any facility, is now required by the NEC for critical operations power systems (i.e., MCFs).


Standard for Safety for Surge Protective Devices

Initially, UL 1449, the Standard for Safety for Surge Protective Devices, was intended as a standard for safely installing surge protective devices in facilities. As this standard evolved over the last three editions, the content has changed to include performance standards of surge protective devices based on specific wave test criteria. These criteria define the specific test conditions under which a device must be tested; the voltage, current, number, and duration of the test impulses; and the peak, let-through voltage that is permitted to pass these tests. So now, the engineer is able to specify a surge suppressor with a level of confidence that it really will limit the voltage to the mission critical equipment to a specific level. In addition to the testing changes, UL 1449 has been accepted as an ANSI standard called ANSI/UL 1449-2006, giving it much broader acceptance as a manufacturing and testing criteria.

References

“A Low-Cost Insurance Policy (Surge Suppressors),” Kenneth L. Lovorn, CSE, June 2002



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