NERC TPL-007-1: Geomagnetic Disturbances and What They Mean for Your Large Power Transformers
Although extremely unlikely, in a worst-case scenario solar storms could disrupt the electric grid and leave millions without power.
Severe geomagnetic disturbances (GMDs) caused by solar storms enter through the upper layers of our atmosphere and can induce currents in long conductors, such as power lines, on the Earth’s surface. This could overload the electric grid system triggering voltage collapse -or worse -damage expensive extra-high voltage (EHV) power transformers.
Protecting the bulk electric system from the impact of these GMDs is the focus of the new North American Electric Reliability Corporation (NERC) standard TPL-007-1.
How Geomagnetic Disturbances Can Impact Transformers
Large power transformers connected to the EHV transmission system may experience both winding and structural hot-spot heating due to GMDs. The primary impact on EHV power transformers is a result of the quasi-dc current that flows through wye-grounded transformer windings. This geomagnetically-induced current (GIC) causes an offset of the ac sinusoidal flux in the transformer core, resulting in asymmetric or half-cycle saturation which leads to a number of effects including:
- Increased exciting current and reactive power absorption
- Hot spot heating of non-current carrying metallic members due to stray flux
- Harmonics
- Increased vibration and noise level
- Hot spot heating of windings due to harmonics and stray flux
New NERC Standards to Address Geomagnetic Disturbance Events
To adhere to NERC standard TPL-007-1, utilities must conduct both initial and ongoing assessments of the potential impact of a “1-in-100-year” benchmark GMD event on their equipment and the bulk power system as a whole, including:
- GIC Disturbance Vulnerability Assessment for the system’s ability to withstand a benchmark GMD event without causing a wide area blackout, voltage collapse, or transformer damage. (Applicability: planning coordinators, transmission planners)
- Transformer Thermal Impact Assessment to ensure that all high-side, wye grounded > 200kV transformers can withstand thermal transient effects associated with a benchmark GMD event. (Applicability: generation and transmission companies)
The standard also requires corrective action mitigation planning to protect against instability and cascading failures.
Transformer Thermal Impact Assessment
The effects of asymmetric or half-cycle saturation on EHV transformers are relatively well understood but difficult to quantify. Based on the benchmark GMD event, a transformer thermal impact assessment should contain the following analysis:
- Calculation of transformer magnetizing current for the specified GIC currents
- Calculation of peak magnetizing current as a function of GIC level
- Calculation of reactive power consumed by the transformer as a function of GIC level
- Calculation of harmonic components of magnetizing current due to GIC
- Calculation of top clamp, tie plate and winding hotspot temperatures
- Calculation of thermal capability curves for base and peak GIC levels
What Do I Do Next? GMD Assessments on Service-Aged Transformers
Doble’s Professional Services team can perform an engineering study to establish power transformer capabilities while under GMDs according to the IEEE C57.163-2015 Guide.
For more modern transformers, the majority of study input data may be obtained from design review documentation.
On older transformers (> 20 years) design review documentation may not be readily available. The original design information might be available in the marketplace from a new owner of the OEM information in some instances. In other cases, partial study input data can be extracted from test reports and outline drawings while the remaining input data would need to be approximated from sound engineering assumptions in alignment with the IEEE guide.
Interested in learning more?
Take a look at our Geomagnetic Disturbance Risk Mitigation Services page for more information on this topic.
