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Medium voltage (MV) switchgear and transformers play a critical role in the complex electrical distribution networks of large service provider or colocation data centers. They are the connection point to the utility so the architectural design choices and equipment selection for them can have a major impact on the overall cost and reliability. A sometimes overlooked system design aspect is mitigating the potential impact of vacuum circuit breaker (VCB) switching on the MV transformers. Under the right – or should I say “wrong” – conditions, the opening and closing of VCBs can create voltage transients that may damage MV transformers. A global team of electrical distribution experts from Schneider Electric, who I’ve written about before, recently published a paper on the subject. Authored by Tony Parsons, PhD., a senior power system engineer and technical consultant at Schneider Electric, the paper provides background on the nature of the transient problems and it describes factors that might put your transformers at risk. Several common solutions are discussed along with some of the pros/cons of each solution type. The paper does a great job of simplifying a complex topic in a concise manner.

Key Factors Driving the Risk for Damage

Transformer resonance-based failures are difficult to predict for a given installation or for a specific switching event. However, based on experience, there are several factors that may present added risk, including:

• MV vacuum circuit breaker switching
• Relatively short cable lengths (< ~100m) between the VCB and the primary transformer
• Switching of low currents (magnetizing or load current levels)
• Load power factor (magnitude of transients increases with inductive loads)

How to Protect from Voltage Transients

Voltage transients often referred to as “power surges” or “spikes”, are short, sub-cycle power system disturbances characterized by significantly higher-than-normal voltage levels. These can come from outside (e.g. lightning strikes) or from within a building (e.g. starting up large motor loads, VCB switching). Protection against voltage transients is normally provided by ensuring that equipment has adequate insulation ratings (BIL rating) and through application of external surge-protective devices (SPDs).

SPDs help dissipate transient energy before it reaches distribution or load equipment. Modern-day MV surge arresters are metal-oxide varistor (MOV)-based devices 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.

Why Additional Protection May be Required

While BIL levels and surge arresters provide effective protection in many cases, there are some switching scenarios that may require additional protection, leading 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 transients. This can help enhance protection for transformers against switching transients.

The paper goes into some detail on how voltage transients are produced when vacuum circuit breakers open through a phenomenon called “current chopping” and “reignition”. It further explains that transients can also occur from an action known as “pre-strike”, which can happen when these breakers close. High-magnitude transients can cause damage if the peak voltage exceeds the transformer BIL rating, particularly in the first few turns (i.e., the “end turns”) of the transformer winding. The interesting thing is that the concern is not just with the magnitude of the transient, but also its frequency. Both must be taken into account when developing a protection strategy. 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. One of the difficult aspects of protecting transformers against switching transients is that there can be cases where the transient voltage at the transformer terminals remains within the transformer’s BIL rating. Therefore, it is 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 address transient frequency as well as the transient magnitude.

Four ways to prevent switching transients from damaging your transformers

1. The most commonly specified solution today is the previously described R-C Snubber. Placed between the VCB and the transformer primary terminals, snubbers 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, snubbers installed on the primary side of the breaker or on the secondary side of the transformer are not effective. Note that snubbers do add some cost, take up additional footprint and add some amount of heat that must be accounted for.
2. Specially-designed transformershave also emerged on the market that are resistant to switching transients. Manufacturers claim that snubbers are no longer needed, however, little documentation substantiates these claims yet. Nonetheless, it’s worth investigating as technologies improve and evolve.
3. Another possible solution is to make operational changes and avoid or reduce the amount of switching times when at low loads. The extent to which you can do this will depend on equipment selection and layout.
4. Finally, higher BIL ratings and SPDs can be used to minimize or reduce the risk of such failures. Using a transformer with a higher BIL rating along with conventional surge arresters 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.

Deploy These Strategies to Protect the MV Transformers in your Data Center

Explore the full paper, Switching Transients and Surge Protection for MV Transformers in Data Centers for more detail on this topic. If you’d like to have a discussion with the Schneider Electric engineering team that wrote the paper, comment below.

Check out other blog posts from the Data Center Science Center team.

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