Document C-15 Procedures for Geomagnetic Disturbances Which Affect Electric Power Systems Approved by the Task Force on Coordination of Operation on April 10, 1989 Revised: May 23, 1989 Revised: September 1, 1989 Revised: February 10, 1993 Revised: January 21, 1997 Reviewed: March 25, 1998 Revised: November 7, 2000 Revised: October 15, 2003 Reviewed: August 18, 2005 Revised: January 11, 2007 Revised: March 18, 2016
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Document C-15
Procedures for Geomagnetic Disturbances Which Affect
Electric Power Systems
Approved by the Task Force on Coordination of Operation on April 10, 1989
Revised: May 23, 1989
Revised: September 1, 1989
Revised: February 10, 1993
Revised: January 21, 1997
Reviewed: March 25, 1998
Revised: November 7, 2000
Revised: October 15, 2003
Reviewed: August 18, 2005
Revised: January 11, 2007
Revised: March 18, 2016
NPCC Document C-15 Procedures for Geomagnetic Disturbances
Which Affect Electric Power Systems
Page 2 of 18
Procedures for Geomagnetic Disturbances
Which Affect Electric Power Systems
Table of Contents
1.0 Introduction 3
2.0 Impacts on Power Systems 3
3.0 NPCC Alerts of Geomagnetic Disturbances 6
4.0 Recommended Procedures 7
4.1 Operational Planning 7
4.2 Operator Action With the Onset of a GMD 8
5.0 Back-Up Communications 8
5.1 Communications Failure 8
5.2 Conflicting Data 9
Appendices
A Communication Paths
B Solar Alerts Issued by the Solar Terrestrial Dispatch
C Solar Alerts Issued by the Space Weather Prediction Center of
the National Oceanic and Atmospheric Administration (Boulder,
Colorado)
D Solar Alerts Issued by the Geological Survey of Canada,
Department of Natural Resources Canada (Ottawa, Ontario)
E Time Conversion Reference Document
F Description of Geomagnetic Disturbances
Note: Terms in bold typeface are defined in the NPCC Glossary of Terms
NPCC Document C-15 Procedures for Geomagnetic Disturbances
Which Affect Electric Power Systems
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1.0 Introduction
The sun emits streams of charged protons and electrons known as solar wind.
The intensity of the solar wind is determined by sunspot activities (solar flares,
coronal holes and coronal mass ejections). The charged solar energetic
particles that escape the sun’s halo (corona) interacts with the earth's magnetic
field producing auroral currents that follow circular paths around the earth's
geomagnetic poles. These non-uniform currents then cause time-varying
fluctuations in the earth's magnetic field, which in turn induce a potential
difference on the surface of the earth (Earth Surface Potential) and result in
Geomagnetically Induced Current (GIC). GIC is a quasi-dc current, frequency
that is less than 1 Hz, which enters and exits the power system at transformer
neutral grounds.
The Earth Surface Potential is measured in volts per kilometer and its
magnitude and direction are functions of the change in magnetic field, Earth's
resistivity (composition of soil/crust), and geographic latitude. Earth surface
potential increases with increasing latitudes and its gradient is highest on
facilities having an east-west orientation. Earth surface potential is highest in
igneous rock areas and where transmission lines terminate near water. Due to
the Earth Surface Potential being greater at higher latitudes, areas with close
proximity to the Earth's magnetic north pole typically experience greater
effects of GMDs. However, a severe storm can affect equipment and systems
even at lower latitudes.
NERC has implemented EOP-010 which requires Reliability Coordinators and
applicable Transmission Operators to develop Operating Plans, Procedures,
and Processes to mitigate the effects of GMD.
Those utilities most affected by solar activity since 1989 have developed
procedures which establish a safe operating posture and which are initiated by
criteria for their respective systems.
A detailed technical description of GMDs is found Appendix F.
2.0 General Impacts on Power Systems
The flow of GICs in the transmission system may affect the following equipment:
2.1 Power Transformers
The presence of GIC produces off-setting dc excitation in a
transformer, resulting in some degree of half cycle core saturation.
Several effects that result from half cycle saturation are:
Creation of harmonic currents
NPCC Document C-15 Procedures for Geomagnetic Disturbances
Which Affect Electric Power Systems
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o Can distort system voltages and cause protective relays
to misoperate due to the GIC flow in the neutral current
to ground.
Increased VAr consumption
o Saturated transformers are reactive power sinks,
consuming system reactive capacity, resulting in voltage
depression.
Increased audible noise
o The characteristic hum of a transformer will become
noticeably louder in a transformer that is exposed to
actual GIC
Tank and winding heating
o Core saturation can also result in internal localized
heating of the core and windings and degradation of
winding insulation.
Increased transformer gases
o Due to increased heating causing the degradation of
winding insulation, transformer gases will be produced.
2.2 HVdc Systems and Static VAr Compensators
When GIC is present in the transmission system, half cycle saturation
may occur. High Voltage dc (HVdc) facilities are more susceptible to
commutation failure. This is due to half cycle saturation and harmonic
currents inducing a non-sinusoidal signal. This non-sinusoidal signal
has relatively high peaks. HVdc systems require a clean sinusoidal
voltage signal to properly commutate current transmission. Operations
at or near the minimum or maximum current rating of HVdc circuits
increases the potential for commutation failures, jeopardizing
continuity of service. Ac voltages distorted by harmonic currents can
also cause filter banks in HVdc facilities to shut down.
Filter banks, including capacitor banks, associated with these systems will
tend to overload due to harmonic current and may result in tripping.
2.3 Shunt Capacitor Banks
Shunt capacitor banks can overload due to harmonic currents experienced
as a result of half cycle saturation. Factors such as grounding of the bank,
relay protection schemes, and impedance that the bank sees from the
transmission system may result in a capacitor bank being susceptible to
tripping.
2.4 Generators
NPCC Document C-15 Procedures for Geomagnetic Disturbances
Which Affect Electric Power Systems
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Automatic voltage regulators (AVR) associated with generators require
representative AC voltage signals to control the DC field current on the
generator exciter. Distorted AC voltage inputs to the AVR result in a poor
feedback signal to the DC field control circuit. This unstable signal may result
in a cyclical level of excitation on the generator and hence real and reactive
power output may vary in an abnormal manner. Overheating may occur in large
generators due to imbalances in phase currents and harmonic distortion in
voltages which result from the saturation of power transformers. Turbine
mechanical vibration may be excited by the presence of increased harmonic
rotor current.
2.5 Transmission Lines
Transmission Lines are mostly affected by harmonics produce by saturated
transformer and less by GIC circulation. Harmonics in the transmission
system can increase the magnitude of the voltage required to be switched
by circuit breakers. This voltage can exceed the circuit breakers’ ratings.
Harmonics increase transmission line losses as well as causing interference
to relay communications systems. This interference can cause inadvertent
tripping.
2.6 Protection and Control circuits
Electromechanical relays are more susceptible to misoperation as a result of
GIC. This can be attributed to the lack of filtering these devices have. New
microprocessor based controllers employ filtering to the inputs used by the
device which can diminish the impact of distorted signals when exposed to GIC.
Both types of devices depend on current transformers (CT). These CTs are
susceptible to saturation, but the microprocessor based relays are less likely to
trip as a result of the harmonics induced. During a GMD event, protection and
control devices may experience elevated harmonic content and increased risk of
current transformer (CT) saturation.
Incorrect operation of protection and control devices can lead to unintended
isolation of equipment such as transmission lines, transformers, capacitor banks
and static VAr compensators, thereby reducing margins and potentially moving
the system closer to collapse.
2.7 Overall System Impact
Transformer saturation results in increased VAr consumption and harmonic
injection into the system. These harmonic currents can result in capacitor bank
overloading and their tripping, generator tripping and misoperation of static
VAr compensators. This could further deplete the system of reactive VAr
support and impact the overall system performance and security. The power
NPCC Document C-15 Procedures for Geomagnetic Disturbances
Which Affect Electric Power Systems
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systems are becoming more vulnerable to GIC effects due to longer
transmission lines, decreased reactive margins and greater dependence on static
VAr compensators and high voltage dc control.
3.0 NPCC Alerts of Geomagnetic Disturbances
The NPCC Reliability Coordinator (RC) Areas receive, on a continual twenty-
four by seven basis, the status of solar activity and geomagnetic storm alerts
from the Solar Terrestrial Dispatch (STD). The primary mechanism for
notification to the NPCC RC Areas is the Solar Terrestrial Dispatch’s
Geomagnetic Storm Mitigation System (GSMS), an active communications
software package accessed by the operator. Upon receipt of a geomagnetic
storm alert from Solar Terrestrial Dispatch of level Kp 6 or higher, the GSMS
simultaneously displays:
• a box advising the operator of a “Major or Severe Geomagnetic Storm
Warning.” At the discretion of the RC Area, audible warnings will
also accompany the flashing dialog box. Both the flashing indicator
and the audible warning (if utilized) will continue until the “Confirm
Receipt of Alert” button is clicked in the dialog box. This sends a
verification to Solar Terrestrial Dispatch that the storm alert was
received by the NPCC Areas. After acknowledging receipt of the
geomagnetic storm alert, the GSMS will immediately prompt the
operator to specify any GIC activity that has been observed on the
power grid. The operator responds by selecting the strength and
entering the location on the GIC Report tab in the software.
• a main screen providing the operator with all information currently
known about possible solar activity. The following information is
presented:
• the time of the notification in both Universal Time and the RC Area’s
local time in twenty-four hour format
• the history of actual hourly Kp readings for the previous
seven hours
• the Kp prediction, together with the predicted time of its onset,
and the predicted peak Kp reading for the next twenty-four, forty-
eight and seventy-two hours periods
• the maximum Kp prediction for both the auroral and sub-
auroral zones
NPCC Document C-15 Procedures for Geomagnetic Disturbances
Which Affect Electric Power Systems
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• the probability of occurrence of the predicted Kp level
• the probable duration of the geomagnetic storm
• a graphical presentation over time of both current historical
Kp observations and projected Kp values
• the status of the receipt of the notification by all NPCC RC Areas.
After reviewing the available data, the operator may choose to enact protective
measures. Such protective measures taken are to be promptly reported as soon
as possible to all NPCC RC Areas. The method for reporting to all NPCC RC
Areas is prescribed by the entities internal procedures or processes. Upon the
halting of a previous action taken, the operator is to similarly report these steps
to all NPCC RC Areas.
In the event that an RC Area observes GIC activity absent the notification of a
geomagnetic storm alert, the operator is to use the “GIC Reports” feature of
GSMS to automatically notify the other NPCC RC Areas and the Solar
Terrestrial Dispatch of the strength and location of the GIC activity.
All time alerts issued by the Solar Terrestrial Dispatch are disseminated in
Universal Time (Greenwich Mean Time), a constant scientific time reference.
All references to Universal Time may be converted to the prevailing Eastern
Time or Atlantic Time as described in Appendix E.
A summary of the levels of solar activity that are made available to the NPCC
RC Areas by the STD are shown in Appendix B. Further details can be obtained
through the Web site of Solar Terrestrial Dispatch at:
http://www.spacew.com/
4.0 Recommended Procedures
4.1 Operational Planning
On receiving from the Solar Terrestrial Dispatch a forecast of GMD activity
expected to result in Kp levels 7, 8 or 9, operations should be reviewed for
vulnerability to such storms. Actions should be considered which include, but