switching transients and surge protection for mv

Switching Transients and Surge

Protection for MV Transformers in Data Centers

Executive summary Voltage transients in MV power systems have been observed to contribute to failures of both power and instrument transformers in data centers in recent years. This white pa-per provides a background regarding the na-ture of the transient problems, as well as a discussion of factors that may put transform-ers at risk. Several common solutions are available to help safeguard transformers, and each of these is discussed along with some of the pros/cons of each solution type.

Revision 0

White Paper 276

by Antony Parsons, Ph.D., P.E.

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Schneider Electric – Data Center Science Center White Paper 276 Rev 0 2

Switching Transients and Surge Protection for MV Transformers in Data Centers

Voltage transients (see sidebar), often imprecisely referred to as “power surges” or

“spikes”, are short, sub-cycle power system disturbances characterized by signifi-

cantly higher-than-normal voltage levels. Though transients do not last very long –

their typical duration may only be a small fraction of a cycle – they can potentially

cause improper operation of sensitive loads or damage to components due to destruc-

tion of insulation. Figure 1 shows an example of a voltage transient that may be pro-

duced when a vacuum circuit breaker (VCB) operates to interrupt a highly inductive

current. Note the time scale – the peak voltage is reached within 150 microseconds,

or less than 1/100th of a 60Hz cycle and the entire event is over within approximately

400 microseconds.

Significant voltage transients may not be result from every switching operation, but

protection should be provided in cases where they are more likely. Protection against

transients is normally provided by ensuring that equipment has adequate insulation

ratings and through application of external surge-protective devices (SPDs). The in-

sulation rating most relevant to voltage transient protection is the basic insulation level

(BIL), sometimes referred to as the lightning-impulse rating. The BIL rating is the

peak voltage rating of the equipment insulation, and is usually several times higher

than the nominal RMS system voltage. SPDs provide additional protection by helping

to dissipate transient energy before it reaches distribution or load equipment. Modern-

day medium-voltage (MV) surge arresters are metal-oxide varistor (MOV)-based de-

vices that conduct surge energy to ground once the applied voltage exceeds a certain

level. They may be applied at service-entrance switchgear, at sensitive loads, and/or

at other key points in the power distribution system.

While BIL levels and surge arresters provide effective protection in many cases, there

are some specific switching scenarios that may require additional protection. This

has led to increased use of R-C snubbers to protect power transformers in data center

facilities. (An R-C snubber, essentially a series resistor and capacitor connected from

each phase to ground, helps to filter out high-frequency content from switching tran-

sients. This can help enhance protection for transformers against switching transi-

ents.) This paper will provide an introduction to switching transients that may result

in excessive transformer intra-winding stress, a discussion of system conditions that

can be potentially problematic, and a summary and comparison of available solutions.

Introduction

Figure 1 Example voltage transient produced by a vacuum circuit breaker interrupt-ing an inductive circuit

Voltage transients can be caused by external sources (e.g., lightning strikes to the system), or internal sources (e.g., circuit breaker switch-ing). In this white paper, we will focus primarily on switching transients.

2 types of voltage transients:

Impulsive transients are unidi-rectional in polarity (either posi-tive or negative) and may be produced by lightning strikes. Oscillatory transients vary rapidly in magnitude (positive and negative) and may be pro-duced by switching. Both types can be potentially damaging.



While voltage transients can be problematic for any type of power distribution equip-

ment, issues with power transformers and voltage transformers caused by switching

transients produced by VCBs have received increased attention in recent years.

Some of the issues observed have been failures internal to the transformer winding

due to intra-winding resonance. While some believe this is a relatively new issue, it

is not – Greenwood1 addresses failures related to transformer internal winding reso-

nance dating back to the early 1970s. More recent attention has led to several tech-

nical papers addressing the issue, as well as development of an IEEE standard2.

Switching transients are produced as a byproduct of normal MV circuit breaker switch-

ing, both on opening and closing of the breakers. Opening an MV VCB may result in

current chopping and reignition, which can produce elevated voltages in the system.

Breaker closing may result in pre-strike that can have much the same impact.

When the contacts of a VCB open, the current flow is not immediately interrupted.

Instead, an arc is drawn between the breaker contacts, and current continues to flow

until the sinusoidal current waveform passes through a current zero. It is not normally

until this point that the arc is extinguished; however, if the current being interrupted is

low, the arc is unstable and may collapse before the current goes through the zero

point. This sudden decrease in current from a few amps to zero – known as current

chop – produces a transient voltage. The frequency and magnitude of the voltage

transient produced depend on the magnitude of the chopping current, as well as the

inductance and capacitance of the circuit. Current chopping is less likely when higher

magnitudes of current are interrupted – either high-level fault currents or even load

current levels.

If the Transient Recovery Voltage (TRV) generated across the breaker contacts during

current interruption exceeds the dielectric strength (i.e., the ability to resist current

flow) of the opening breaker contacts, the current across the contact gap can re-ignite

or restrike. This can cause a high-frequency (100-200kHz) oscillating current through

the circuit breaker. The VCB then interrupts this high-frequency current, but if the

resulting TRV again exceeds the dielectric strength of the separating contacts, it may

restrike again, and this may happen several times before the final interruption. The

voltage in Figure 1 is an example of voltage escalation caused by multiple interrup-

tions and reignitions in a VCB.

Pre-strike refers to a condition during a closing operation where the gap between the

breaker contacts breaks down and arcing occurs just before they are actually closed.

Either way, these conditions can also result in sudden changes in current and a cor-

responding transient overvoltage. Figure 2 shows an example of pre-strike voltage

and current during a VCB closing operation.

1 Greenwood, Allan, Electrical Transients in Power Systems, 2nd ed., John Wiley & Sons, Inc., NY, NY,

1991.

2 IEEE C57.142-2010, IEEE Guide to Describe the Occurrence and Mitigation of Switching Transients Induced by Transformers, Switching Device, and System Interaction.

Switching transients and transformers



A single current chop may not cause problems. Voltage escalation resulting from

repeated chopping and reignition can produce a series of high-frequency voltage tran-

sients with increasing magnitude. Both the magnitude and frequency of the voltage

transient are important. High-magnitude transients can cause damage if the peak

voltage exceeds the transformer BIL, particularly in the first few turns (i.e., the “end

turns”) of the transformer winding.

The frequency of the transient is also important because the electrical characteristics

of the transformer itself are very different for high frequencies than at lower power

frequencies of 50 or 60Hz. Transformer characteristics such as turn-to-turn or turn-

ground capacitances can be neglected at 50 or 60Hz, but have a more significant

impact at high frequencies. The impedance of the transformer is not constant with

frequency, but instead has resonant points at various frequencies, as shown in Figure

3 from IEEE C57.1422. A transient voltage does not operate at a constant frequency,

either – instead, the sudden change in voltage during a transient (i.e., the fast dv/dt)

produces voltage over a relatively wide range of frequencies. The typical VCB switch-

ing transient has frequency content ranging from 50-100kHz. If the transformer has

resonant frequencies in this range, the switching transient can “excite” the internal

resonances of the transformer, with potentially damaging results.

As discussed previously, a high-magnitude transient would pose a problem for the

“end windings” of the transformer. A high-frequency switching transient does not just

pose a problem for the “end windings” of the transformer, though, as the non-linear

frequency response of the transformer can result in amplification of voltages at

points within the winding. That is, the voltage at a point internal to the transformer

winding can be higher than the applied voltage at the transformer terminals, as Wag-

Figure 2 Transient voltage (top) and current (bottom) during a pre-strike event.

Figure 3 Typical transformer input impedance vs. frequency, from IEEE C57.142



ner3 demonstrated through controlled testing of switching transient activity with a typ-

ical 15 kV dry-type transformer. See the plots in Figure 4 for an example of this

phenomenon, sometimes referred to as intra-winding resonance.

The voltage levels shown in Figure 4 are relatively low because reduced applied volt-

age levels were used during testing to avoid catastrophic damage to the test trans-

former. The peak measured voltage at the transformer center tap was still 6X the

peak transient value at the transformer terminals, and 20X the nominal applied volt-

age. This is one of the difficult aspects of protecting transformers against

switching transients – there can be cases where the transient voltage at the

transformer terminals remains within the transformer’s BIL rating, and therefore

below the point at which a conventional surge arrester would provide significant

protection, while voltages internal to the winding can rise to levels that can

cause insulation failure. Protection for power transformers, therefore, must ad-

dress transient frequency as well as the transient magnitude.

There have also been cases observed where current chopping and reignition have

created voltage transients that have resulted in potential (a.k.a., voltage) transformer

(PT) failures. Authors of one case study cite ferro-resonance as the source of the

issues4, while other analyses have indicated that the magnitude of the switching tran-

sient may be sufficient to create issues for PTs. Though these cases differ from the

power transformer cases discussed above, the potential solutions are similar.

The switching transient issue discussed above is a system issue. That is, it is not

strictly related to a “defective” transformer or any other single component. Instead, a

combination of factors must all be present for voltage transients to appear at the

“wrong” frequencies and induce transformer failures. Some of the relevant parame-

ters, such as the detailed frequency response of a given transformer winding or ex-

pected levels of current chop from a given breaker operation, are either not well un-

derstood or are difficult to predict beforehand. As a result, transformer resonance-

based failures are difficult to predict for a given installation or for a specific switching

event. However, based on our experience, there are several factors that may present

added risk, including:

• MV vacuum circuit breaker switching

• Relatively short cable lengths between the VCB and the transformer primary (< ~100m)

3 Wagner, Van. “Experimental Evaluation of Switching Induced Transformer Resonance Mitigation,”

IEEE Transactions on Industry Applications, vol. 54, no. 4, July/August 2018.

4 McDermit, et. al., “Medium Voltage Switching Transient Induced Potential Transformer Failures: Pre-diction, Measurement and Practical Solutions”, IEEE Transactions on Industry Applications, vol. 49, no. 4, July/August 2013.

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System conditions that can lead to equipment failure

Figure 4 Voltage transient at the transformer HV terminals (L) and at the center of the TX winding (R), for a typical 15 kV Dry-Type transformer.



• Switching of low currents (magnetizing or load current levels)

• Load power factor (inductive)

Vacuum breaker switching at MV

As discussed previously, current chopping is required to produce the voltage transient

that can excite an internal transformer resonance, and the highest transient magni-

tude is achieved after a series of reignitions. Switching of a primary VCB is the most

likely source of these phenomena. Of the two predominant MV breaker technologies

– i.e., vacuum and SF6 – vacuum breaker switching produces significantly higher lev-

els of chopping current, while SF6 is not prone to successive reignition. This does not

necessarily mean transformers fed from SF6 breakers are 100% immune from transi-

ent issues, but that issues are much less likely due to the different breaker character-

istics. Note that significant chopping current magnitudes are not produced by MV vac-

uum contactors, vacuum switches, or air switches.

A full discussion of the reasons why is beyond the scope of this paper, though more

information is available in Cahier Technique #1935.

Primary cable lengths

Case studies indicate that switching-induced transformer failures are less likely when

there are longer cable lengths between the MV circuit breaker and the transformer(s).

This is believed to be a result of the inherent MV cable capacitance to ground, since

the capacitance acts as a filter for the high-frequency content in the voltage transient.

Whether the capacitance is provided by a surge capacitor, the capacitive element of

an R-C snubber, or capacitance of the cable itself, the filtering result is the same.

The exact length of cable required to mitigate transient activity is not well defined.

Shipp6 suggests that issues are more likely with primary cable lengths below 200 feet

(~60m). Empirical evidence suggests that cables greater than 800 feet (~240m) pro-

vide sufficient damping to mitigate transient activity, but this level may vary depending

on cable characteristics. The voltage level of the cable is also a factor to be consid-

ered – the per-unit-length cable capacitance is higher for lower-voltage cables, which

may help to explain why switching-transient induced issues are not as common at 5

kV. Other differences in cable construction or application, such as insulation type,

may also be a factor. In any event, transient issues are more likely for cable lengths

less than approximately 300 feet (~100m).

Load characteristics

Potentially damaging current chopping is most likely when VCBs interrupt relatively

low current levels. As discussed previously, the arc drawn between the separating

breaker contacts during interruption of low currents is not as stable as it is during fau lt

interruption. As a result, the arc can more easily collapse as the current waveform

approaches its zero-crossing. When the arc collapses, this traps energy in the circuit

inductance, which interacts with circuit capacitance to produce the voltage transients

such as those seen in Figures 1, 2, and 4. There have been numerous cases in US

data centers where transformer damage has resulted from switching unloaded trans-

formers – for example, the transients in Figure 4 were the results of switching ~3A of

transformer magnetizing current.

5 Theoleyre, S. “MV Breaking Techniques”, Cahier Technique No. 193, Schneider Electric, Grenoble,

France, 1999.

6 Shipp, D. D., et. al., “Transformer Failure Due to Circuit-Breaker-Induced Switching Transients”, IEEE Transactions on Industry Applications, vol. 47, no. 2, March/April 2011.

While the threshold for MV cable length that helps to at-tenuate switching transients is not well-defined, longer primary cables do make transient damage less likely.

An R-C snubber helps to fil-ter out high-frequency con-tent from switching transi-ents and can be an effective way to protect transformers against switching transients.

https://www.schneider-electric.com/en/download/document/Cahier+Technique+n%C2%B0193/

https://www.schneider-electric.com/en/download/document/Cahier+Technique+n%C2%B0193/



The power factor of the load is important, too. Analysis and testing has shown that

the peak magnitude of voltage transients is lower when the load power factor is near

unity or leading, but that it increases as currents are more inductive. In a data center

transformer feeding server load, a high power factor – perhaps even a leading power

factor – is expected. However, unloaded transformers still draw magnetizing current,

which is highly inductive in nature.

Operational considerations

When a transformer is switched with an upstream VCB rather than by switching a local

disconnect (e.g., MV air switch applied at the transformer primary), a voltage transient

will be produced. The magnitude and potential effects of the transient will depend on

a number of factors. Common switching scenarios such as opening a VCB to simulate

the loss of utility power during data center commissioning puts the transformers at

elevated risk of damage due to switching transients. Minimizing VCB switching re-

duces the risk exposure.

The number of switching operations that may occur can make operational considera-

tions more important. Consider that a transformer’s effective BIL is highest when it

first leaves the factory. The effective BIL is degraded over time by repeated transient

activity, even if the voltage transient magnitudes to which it is subject are below the

nameplate BIL rating. (Think of this as the electrical insulation equivalent of erosion.)

Therefore, the sheer number of switching operations that may occur during data cen-

ter commissioning – on one project, some VCBs feeding transformers were ultimately

switched over 200 times – can be a contributing factor as well.

Additional factors

There are other factors which have been raised as potential issues but that are either

not as clearly correlated to observed issues as those noted above, or that have not

been extensively studied. These include:

Transformer characteristics – there have been more observed instances of switch-

ing-induced failures in dry-type or cast-coil transformers than in liquid-filled transform-

ers. Several possible explanations exist, including inherent differences in transformer

design that may affect the frequency response, differences in standard BIL ratings,

capacitance of the insulating liquid, or even different habits/applications in facilities

where different types of transformers are used. Particularly where differences in

transformer design and the inherent characteristics of the units are concerned, infor-

mal surveys of transformer design engineers over the years have shown there is no

clear consensus on these issues. As such, it is impossible to say that liquid-filled

transformers are immune to the issues discussed in this paper simply because of any

characteristics inherent to the basic transformer design.

Transformer efficiency – evolutions in transformer design requirements from NEMA7

and the US Department of Energy have resulted in improved transformer efficiency

standards, to the point that losses in today’s US transformers are significantly lower

than they were as recently as 15-20 years ago. For example, transformer core losses

have been reduced by a factor of 20X in present designs compared to 1950s designs,

largely through improvements in the core materials being used. Lower resistive losses

may mean less damping for transient activity, possibly increasing the chances that

damage could occur from a given switching event. However, note that even though

these issues are more widely recognized today, similar transformer failures have been

documented well before the recent changes in transformer design were implemented.

7 “Guide for Determining Energy Efficiency of Distribution Transformers,” National Electrical Manufac-

turer’s Association, Arlington, VA USA, Technical Report NEMA TP-1, 2002.

More transient-related fail-ures have been observed with dry-type than with liq-uid-filled transformers, but this does not mean that liq-uid-filled units are immune to the issue.



U.S. vs. International Designs – More of the transient-related failures discussed

here have been observed on power transformer installations in the U.S. than else-

where in the world. The reasons why are not clear. Perhaps differing efficiency stand-

ards do play a part. Perhaps fundamental differences in transformer designs result in

different frequency responses. Perhaps differences in typical operating practices in

facilities plays a part. Or, it may simply be that more problems have been seen in

U.S. data centers because there are more data centers in the U.S.

Despite the attention that this issue has received, many potential contributing factors

are still not well-understood, and so it may be that other issues, not yet clearly identi-

fied, account for the differences. However, the issues described for power or instru-

ment transformers are not strictly a North American problem or ANSI/IEEE equipment

problem – given the right combination of factors, problems are possible regardless of

geography.

Several solutions to the issues discussed are available. In this section, we will discuss

four solutions that should either help make transformers resistant to switching-induced

failures, or that should at least reduce the likelihood of issues in practice.

Snubbers

The most commonly-specified solution for this issue at present is the Resistor-Capac-

itor (R-C) snubber. The snubber consists of a resistor and capacitor in series, con-

nected from phase to ground in-between the VCB and the transformer primary termi-

nals, as shown in Figure 5. The snubber can be made up of separate R & C compo-

nents, or it may be contained within a single, separate enclosure. Though not shown

in Figure 5, a snubber is also provided with fuse protection and may be equipped with

a blown fuse detection system to provide positive indication that the system is oper-

ating.

While some references will indicate that individual snubbers should be applied at the

primary terminals of each transformer, testing performed by Schneider Electric in the

US3 has demonstrated that this is not the case. Instead, as shown in Figure 5, as

long as the snubber is placed between the VCB and the transformer primary terminals,

it will provide the required filtering. Only one snubber is required per MV feeder,

even if the feeder serves more than one transformer. Since the voltage seen at

VCB terminals during switching mostly appears on the load side of the breaker, snub-

bers installed on the primary side of the breaker or on the secondary side of the trans-

former are not effective.

Solutions

Figure 5 Circuit diagram showing possible snubber applica-tion locations. The snubber must be applied between the VCB and transformer.



The snubber unit does require additional mounting space – typically an additional

switchgear or switch section, or a separate enclosure installed at the transformer.

IEEE C57.142 provides some guidance on snubber design. In practice, the required

component ratings are within a wide-enough range that a standard design can be

developed for each voltage class using commonly-available R and C components,

such as surge capacitors. The performance of the snubber solution is shown in Fig-

ure 6. The plots show measured voltage at the transformer primary terminals without

(Left) and with (Right) an R-C snubber applied. The multiple reignitions and high-

frequency content that could excite the transformer internal resonances have been

effectively filtered out by the application of the snubber. Snubber application has

proven effective for mitigation of transients causing issues for instrument transformer

installations as well – the chopping and reignition issues potentially leading to dam-

aging transient magnitudes are effectively mitigated.

Potential drawbacks of snubber applications include increased cost for the snubber

installation, increased equipment footprint, and addition of a new potential point of

failure. Snubbers also produce additional heating, which must be managed, particu-

larly if they are applied inside a switchgear enclosure. Note also that addition of many

snubbers is equivalent to adding a power factor correction capacitor bank to the sys-

tem. This can potentially create harmonic distortion issues both for the system and

for the capacitive elements of the snubbers themselves – i.e., snubbers that uninten-

tionally filter harmonic current from the system can have their capacitive elements fail

prematurely. This may more likely be an issue in systems where standby generators

can feed the system at MV, as the generators would tend to have a lower fault current

availability than the utility source. This increases the probability that a harmonic res-

onance would be created at a frequency where significant harmonic content may al-

ready be present (from VFDs, UPSs, etc.).

Specially-designed transformers

Several transformer manufacturers in the world manufacture power transformers

that are designed to be resistant to the types of switching transients discussed in

this paper. One is a cast-coil transformer with MOV-based surge arresters applied at

selected locations within the transformer windings to help mitigate intra-winding volt-

age transients such as the one shown in Figure 4 (Right). The other is a liquid-filled

unit with elevated BIL ratings and a frequency response characteristic that leaves it

less prone to elevated voltage transient levels due to VCB switching. Latest one is a

dry-type transformer with an elevated BIL ratings, designed to promote linear volt-

age distribution along the coils and with a frequency response characteristic that

avoid internal resonance; this is associated with conventional surge arresters at the

transformer input that provide a full resistant system to both intra-winding voltage

transients as well as the voltage transient magnitude,

All manufacturers claim that the transformers may be applied without snubbers.

Generally, little is publicly available to document performance outside of marketing

literature provided by the equipment vendors. Since products are relatively new to

Figure 6 Voltage transients meas-ured at the transformer HV terminals, without (Left) and with a snubber applied (Right).



the market, long-term reliability is unknown. The internal MOVs in the first design

would be a source of potential concern. MOVs degrade and eventually fail when ex-

posed to transient voltages over time, meaning that at some point the MOV units

would either need to be replaced or would be at danger of failing and leaving the

transformer unprotected. Use of the liquid-filled transformer design would require, at

the least, provision of a liquid confinement area. No “transient resistant” PT designs

are currently available.

Operational changes

Given that VCB switching of unloaded transformers is one of the practices that ex-

poses transformers to danger of damage, modifying switching procedures to eliminate

or reduce the number of times the transformers are switched in this manner is advis-

able. The extent to which the switching philosophy can be changed will depend on

equipment selection and system layout. As such, this is a consideration that should

begin in the design stage of the facility, and may extend through commissioning and

normal operation. Of course, operational changes do not actually solve the problem

at hand; they simply avoid it. If modified switching procedures cannot be followed at

all times, one of the other solutions may be required as well.

BIL ratings

Increasing the BIL ratings of the transformers from the standard levels (e.g., going

from a standard 60kV BIL to optional 95kV or 110kV for a 15kV dry-type transformer)

is likely to be helpful, though this may not be sufficient to completely solve the transi-

ent issue. That is, a transformer with higher BIL in a system with no other considera-

tions made is not going to be immune to failure (particularly over time). Conversely,

a transformer with standard BIL will not automatically fail on the first operation. To

the extent that a higher BIL represents a more robust insulation system overall, it can

be beneficial when combined with other solutions, such as a snubber. Since the ef-

fective transformer BIL should be expected to degrade over time, it is clear that s tart-

ing from a higher level can help to insure a sufficiently strong insulation system over

time. Using a transformer with a higher BIL rating along with conventional surge ar-

resters may be a simple, low-cost solution where site operational practices or absence

of other risk factors mean that exposure to potentially damaging transients is reduced.

Vacuum circuit breaker switching of transformers may produce voltage transients that

can produce catastrophic damage to transformer winding insulation in both power and

instrument transformers. This requires the right combination of factors, including em-

ployment of specific switching procedures, but this combination of factors is not un-

common in the Data Center environment.

Solutions are available, and may be as simple as minimizing exposure to switching

transients by switching transformers with local disconnect switches rather than up-

stream vacuum circuit breakers. Application of snubbers in conjunction with conven-

tional surge arrestors has proven to be effective, and is the primary solution recom-

mended by IEEE Std. C57.142. Transient-resistant transformers are also now avail-

able from some manufacturers. Other factors, including increasing standard BIL rat-

ings of transformers, may also be helpful. Each solution has its own set of pros/cons

which must be considered. They may be applied separately or in combination to help

avoid issues in systems where the “wrong” conditions exist.

Conclusion



About the author

Antony Parsons is a Technical Consultant based in Austin, Texas, USA. He received the B.S.

degree from the University of Houston and M.S.E.E. and Ph.D. degrees from the University of Texas

at Austin, all in Electrical Engineering with a focus on electric power systems. He joined Schneider

Electric in 1999, and has worked as part of the US-based Power System Engineering team ever

since. His areas of expertise include power system analysis, power quality, and electrical safety.

Antony is a member of the IEEE P1584 working group on Arc-Flash Hazard Calculations. He has

authored and presented numerous white papers, magazine articles, IEEE papers, webinars, and

conference tutorials on various aspects of power system analysis and operation.

Acknowledgements Special thanks to Van Wagner for his review and support, which were invaluable in the develop-

ment of this paper.

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