Picture this: a spot erupts on the sun’s surface and creates a geomagnetic storm heading straight for Earth. Global positioning systems are disabled, communication technologies are interfered with and large portions of the grid are down, leaving us in the dark.
While this may sound like a sci-fi movie, it’s actually a real scenario —and one that comes with a new standard (NERC TPL-007-1) and guidelines for protecting the grid.
Solar storms can induce currents in long conductors, such as power lines, which can overheat power transformers and potentially cause voltage collapse.
While these situations don’t occur often, their impact can significantly compromise power systems, which is why they’ve become known as high-impact, low frequency events—on par with natural disasters, cyber-attacks and nuclear power plant accidents.

If a transformer is susceptible to solar flares, what does that mean?

Large power transformers connected to a transmission system may experience excessive winding and structural hot-spot overheating due to these geomagnetic disturbances (GMD). The effects can last up to a couple days. The quasi-DC geomagnetically-induced current (GIC) that flows through wye-grounded transformer windings causes an offset of the ac sinusoidal flux in the transformer core, resulting in asymmetric or half-cycle saturation.
This means your transformers can be subjected to a variety of unwanted effects, including: increased exciting current and reactive power absorption, hot spot heating of non-current carrying metallic members or windings due to stray flux, and the detrimental effects of harmonics, system voltage instability, increased vibration and noise level.

New NERC TPL-007-1 standard

On New Year’s Day, the North American Electric Reliability Corporation (NERC) imposed a new standard to protect the bulk electric system from the impact of GMDs. To comply, utilities and power companies must conduct ongoing assessments of their transformers to measure and monitor the impact of GMD events on the bulk power system.
GIC disturbance vulnerability assessments must be performed to check the system’s ability to withstand the benchmark GMD event (1-in-100-years) without causing a wide area blackout, voltage collapse or transformer damage. Thermal impact assessments are also now required for ensuring that all high-side, wye-grounded transformers above 200 kV can withstand thermal transient effects associated with a benchmark GMD event.
Compliance dates are spread out over the next few years, but as with any new program, the time to start planning is now. Below are a few strategies for a successful GMD risk mitigation program.

• Perform GIC flow studies to determine which transformers may be subject to a GMD benchmark event. A transformer’s viability will depend on the time and magnitude, or signature, of the event. Planning departments should handle this and provide GIC currents and durations at the neutral for engineering study purposes.
• Find evidence of power transformer capabilities under GIC. Check transformer factory acceptance test records – many companies have updated their transformer purchase specifications to include GIC capability studies as part of their procurement process. Asset owners are recommended to confirm original equipment manufacturer (OEM) studies were performed for the right GIC current levels and durations.
• Conduct a thermal impact assessment. Based on GIC signature levels as defined from a company’s system analysis, a thermal impact assessment can help establish power transformer capabilities under GIC disturbances per IEEE C57.163-2015. These rectangular-shaped signatures can greatly simplify the calculations of transformer magnetic and thermal response to GIC and the evaluation can be customized to extend to any required number of events or specified signatures. A thermal impact assessment is required if the GIC is greater than 75 amps and should contain the following calculations:
• Transformer magnetizing current for the specified GIC currents
• Peak magnetizing current as a function of the GIC level
• Reactive power consumed by the transformer as a function of the GIC level
• Harmonic components of magnetizing current due to GIC
• Top clamp, tie plate and winding hotspot temperatures
• Thermal capability curves for base and peak GIC levels
• Think about spare transformer strategies. Plan and review your spare transformer program, which can include: traditional spare transformer strategies, formal sharing arrangements such as EEI Spare Transformer Equipment Programs (STEP) and NERC Spare Equipment Database (SED) programs, or with OEMs and neighboring utilities.
• Perform an optimal transformer condition assessment. Monitoring and measuring the following criteria on an ongoing basis can help teams understand the overall health and vulnerability of their transformer fleet: dissolved gas analysis and oil quality data, electrical test data, external visual inspection, loading history, fault history and maintenance and repair history. By knowing the details of your transformers’ condition, you’ll be better equipped to handle whatever events come your way.

About the authors: Dom Corsi is senior transformer consulting engineer, and Paul Griffin is vice president of global professional services at Doble Engineering Co.