By the North American Electric Reliability Corporation and the Electric Power Research Institute
The highly complex, interconnected North American power grid has provided a long record of reliable, secure delivery of electric power. However, solar storm or geomagnetic disturbance events have demonstrated their ability to disrupt the normal operations of the power grid.
The most recent example in North America occurred in March 1989, when a GMD led to the collapse of the Hydro Quebec system, leaving more than six million people without power for nine hours. Figure ES“1 below shows the March 1989 storm over North America. Understanding the effects of GMD on bulk power systems and the ability of the industry to mitigate their effects are important to managing system reliability.
NERC conducted this assessment in response to findings in the High Impact, Low Frequency Event Risk to the North American Bulk Power System (March 2010) report, which found the best approach to HILF events was through an organized combination of industry-led task forces and initiatives. The GMD Task Force implemented that approach for study of geomagnetic disturbances.
GMD emanate from the sun. According to scientists, solar coronal holes and coronal mass ejections are the two main categories of solar activity that drive solar magnetic disturbances on Earth. CME create a large mass of charged solar energetic particles that escape from the sun’s halo (corona), traveling to Earth between 14 and 96 hours. These high-energy particles consist of electrons, along with coronal and solar wind ions. Geomagnetic induced currents that interact with the power system appear to be produced when a large CME occurs and are directed at Earth.
Charged particles from the CME interact with Earth’s magnetosphere-ionosphere and produce ionospheric currents, called electrojets. Typically millions of amperes in magnitude, electrojets perturb Earth’s geomagnetic field, inducing voltage potential at Earth’s surface and resulting in GIC.
Long manmade conducting paths, such as transmission lines, metallic pipelines, cables, and railways, can act as “antennae” (depending on the impedance), that allow the quasi-DC currents to enter and exit the power system at transformer grounds, disrupt the normal operation of the power system and, in some cases, cause damage to equipment. Current is also induced on the transmission lines through voltage induction on the loop formed by the grounded transmission line and earth. Induction can occur along a loop of transmission lines, which are connected by grounding.
In the U.S., the responsibility for monitoring and forecasting space weather rests with the National Oceanic and Atmospheric Administration Space Weather Prediction Center. In the U.S., magnetometers, which gather data on Earth’s geomagnetic field, are operated by the United States Geological Survey. In Canada, the Canadian Space Weather Forecast Centre is responsible for monitoring and providing services on space weather.
Both North American space weather centers gather the available data in real time describing the state of the sun, heliosphere, magnetosphere, and ionosphere forming a picture of the environment between the sun and Earth. With this information, forecasts, watches, warnings, and alerts are prepared and issued to those impacted by space weather. Scientists and technicians use a variety of ground- and space-based sensors, as well as imaging systems, to view activity at various locations.
There are two risks that result from the introduction of GICs to the bulk power system:
1. Damage to bulk power system assets, typically associated with transformers, and
2. Loss of reactive power support, which could lead to voltage instability and power system collapse.
For extra high voltage (EHV) transformers, the effects of GIC include half-cycle saturation that results in: 1) harmonic currents, 2) fringing magnetic fields (flux that flows outside the core), and 3) increased reactive power (VAr) consumption.
Harmonic currents can cause relays to trip needed equipment; fringing fields can create heating in transformers which, if sufficiently high and sustained for a relatively long duration, can lead to their reduced life; and VAr consumption can cause the system to collapse due to voltage instability. Furthermore, loss-of-life to transformers results from insulation breakdown and can be detected by measuring dissolved gas in the insulating oil.
The magnitude, frequency, and duration of GICs, as well as the geology and transformer design are key considerations in determining the amount of heating that develops in the windings and structural parts of a transformer. The effect of this heating on the condition, performance, and insulation life of the transformer is also a function of a transformer’s design and operational loading during a GMD event.
For example, the failure of the Salem No.1 Nuclear Generator Step-Up Unit shell-type transformer attributed to the March 1989 GMD storm, was due to the development of high circulating currents in the series connections of the low-voltage windings.
Replacing failed EHV transformers is not a small undertaking, as it may require long lead time for design, engineering and manufacturing, unless a spare transformer is located nearby. The loss of a few EHV transformers (greater than 345 kV on the high side) – either closely located or more distant – would rarely challenge bulk power system reliability.
The most likely consequence of a strong GMD and the accompanying GIC is the increase of reactive power consumption and the loss of voltage stability. The stability of the bulk power system can be affected by changes in reactive power profiles and extensive waveform distortions from harmonics of alternating current (AC) from half-cycle saturated high voltage transformers.
The potential effects include overheating of auxiliary transformers, improper operation of relays, and heating of generator stators, along with potential damage to reactive power devices and filters for high-voltage DC lines.
GIC can lead to half-cycle saturation of power transformers and generate significant amounts of odd and even harmonic distortions in the system current and voltages. When transformers are half-cycle saturated from GIC, protection and control devices may experience elevated harmonic distortion and increase the risk of current-transformer saturation, which can lead to incorrect or undesired operation of protection and control devices unintentionally isolating equipment at times when it provides critical support to the system.
Isolating components, such as transmission lines, transformers, capacitor banks and static VAr compensators (SVCs), may reduce margins further, moving the system closer to voltage collapse.
Devices such as SVCs and capacitor banks are also vulnerable to harmonics if the protection device operates on peak or root-mean-square quantities, instead of only fundamental quantities. These reactive power devices are critical to maintaining system stability during GMD events when VAr demand is high.
Restoration times of the power system from these two risks are significantly different. For example, restoration times from system collapse due to voltage instability would be a matter of hours to days, while replacing transformers requires long-lead times (a number of months) to replace or move spares into place, unless they are in a nearby location. Therefore, the failure of a large numbers of transformers would have considerable impacts on portions of the system.
A number of systems in North America have GMD event operating procedures that are triggered by forecast information and/or field GIC sensors. However, NERC’s May 2011 background document, Preparing for Geomagnetic Disturbances alert, indicates “severe GMD events present risks and vulnerabilities that may not be fully addressed in conventional bulk power system planning, design and operating processes.”
Existing operating procedures generally focus on adding more reactive power capability and unloading key equipment at the onset of a GMD event. However, more tools are needed for planners and operators to determine the best operating procedures to address specific system configurations.
Harmonic overloading of SVCs and capacitor banks, at a time when reactive compensation needs are high due to reactive power absorption from transformer half—cycle saturation, can make maintaining system voltage problematic.
Extensive monitoring and simulation models are not widely available, and therefore, the existing operating procedures may not be sufficient to respond to large GMD events. This report recommends development of operational planning and operator visualization tools to enhance situational awareness of GMD impacts.
Significant work in the past two decades has been devoted to the modeling of GIC flows in a high voltage power network. However, modeling the effects on a power apparatus (e.g., transformers) and system performance (e.g., voltage stability) during a GMD event are not as well developed.
GIC flows are highly dependent on the power system’s electrical characteristics and geology. Modeling GIC flows is vital to permit planners and operators to evaluate appropriate combinations of mitigation equipment and operational procedures.
This report provides modeling suggestions to guide power system engineers studying GIC to ensure planning of the bulk power system for a GMD event provides sufficient system margin needed by operators to maintain the reliability of the bulk power system.
An essential part of a GIC mitigation program is the installation of monitors to measure GICs and harmonic currents on a continuing basis. Monitors are a key source of real-time information that can guide system operators in determining real-time response. Additionally, monitors can provide valuable historical records that can be evaluated and factored into power system planning and analysis. Coupled with alerts and warnings issued by the SWPC or CSWFC, monitors can provide the reinforcing information that a GMD event is imminent or in progress and can support operational decisions and actions.
One potential mitigation approach is to reduce GIC flow through the use of series compensation on the line, and/or placing blocking capacitors or neutral resistors in the transformer’s neutral-to-ground connection. This report describes how such devices function, summarizes considerations for their appropriate placement, discusses their failure modes, and summarizes general functional requirements.
The most likely worst-case system impacts from a severe GMD event and corresponding GIC flow is voltage instability caused by a significant loss of reactive power support simultaneous to a dramatic increase in reactive power demand.
Loss of reactive power support can be caused by the unavailability of shunt compensation devices (e.g., shunt capacitor banks, SVCs) due to harmonic distortions generated by transformer half-cycle saturation. Noteworthy is that the lack of sufficient reactive power support, and unexpected relay operation removing shunt compensation devices was a primary contributor to the 1989 Hydro Quebec GMD-induced blackout.
NERC recognizes that other studies have indicated a severe GMD event would result in the failure of a large number of EHV transformers. The work of the GMD Task Force documented in this report does not support this result. Instead, voltage instability is the far more likely result of a severe GMD storm, although older transformers of a certain design and transformers near the end of operational life could experience damage.
The most significant issue for system operators to overcome a severe GMD event is to maintain voltage stability. As transformers absorb high levels of reactive power, protection and control systems may trip supporting reactive equipment due to the harmonic distortion of waveforms. In addition, maintaining the health of operating bulk power system assets during a geomagnetic storm is a key consideration for asset managers.
The magnitude, frequency, and duration of GIC, as well as the geology and transformer design are key considerations in determining the amount of heating that develops in the windings and structural parts of a transformer. The effect of this heating on the condition, performance, and insulation life of the transformer is also a function of a transformer’s design and operational loading during a GMD event. This report reviews past transformer failures from strong GMD events and illustrates that some older transformer designs are more at risk for experiencing increased heating and VAr consumption than newer designs. Additionally, transformers that have high water content and high dissolved gasses and those nearing their dielectric end-of-life may also have a risk of failure.
Planners and operators require the technical tools to model GIC flows and subsequent reactive power losses to develop mitigating solutions, as necessary. This tool development includes GIC flow calculations for a variety of system conditions and configurations, test waveforms representative of GMD for a variety of latitudes, and suitable transient and thermal equipment models. NERC, in collaboration with Electrical Power Research Institute (EPRI), will follow the work plan established in the recommendations section of this report. All results of will be open source and freely available. As work progresses to identify specific vulnerabilities, all assumptions and methods used for planning and operating studies need to be available, transparent, and validated through existing interconnection reliability modeling groups.