Disturbances. Their Impact on the Power Grid. By the IEEE Power & Energy Society Technical Council Task Force on

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1 By the IEEE Power & Energy Society Technical Council Task Force on Geomagnetic Disturbances nasa/sdo/aia Geomagnetic Disturbances Their Impact on the Power Grid Digital Object Identifier 1.119/MPE Date of publication: 19 June 213 GGeomagnetic Disturbances (GMDs), Geomagnetic induced currents (GICs), and their impacts on bulk power systems have been of interest to power engineers since the 196s. However, interest has been heightened recently, and articles in The Wall Street Journal, National Geographic, and IEEE Spectrum have predicted widespread transformer failures and prolonged global power outages. While these articles provide thought-provoking perspectives, they have not offered a scientific and engineering analysis of the many complex issues that determine power system impacts. The purpose of this article is to: provide a technical review of GMDs/GICs, with emphasis on impacts to the power grid and power transformers review detection and measurement of GMDs/GICs and how information is communicated to grid operators discuss various approaches to mitigate the potential impacts on power systems describe industry efforts for technological advancement to address GIC issues. july/august /13/$ IEEE ieee power & energy magazine 71 11mpe4-pestaskforce indd 71 6/6/13 3:11 AM

2 Transformer Bank EHV/UHV Transmission Line GIC Transformer Bank Transmission Line Tower GIC Geoelectric Field GIC figure 1. GIC flow in a power network. Background on GMD and GIC and Its Interactions with Power Grids GMD is a naturally occurring phenomenon initiated by solar activity. Sunspots (relatively cool areas shielded by complex magnetic fields) can give rise to solar flares and coronal mass ejections (CMEs). The CME carries its own currents and magnetic fields that are capable of affecting the Earth s magnetic field. Charged particle movement in the conductive ionosphere increases the current flows in the electrojets, which are currents in the order of millions of amperes located more than 1 km above the Earth s surface. These electrojet currents induce quasi-dc voltages in transmission lines that, in turn, drive the flow of GICs wherever there is a path for them to flow, which is depicted in Figure 1. GICs are often called quasi-dc currents because of their generally low frequency (.1 mhz.1 Hz), and thus network response is essentially resistive. A GMD event can last one to two days and continually generates relatively low to moderate levels of GICs with several intermittent periods of high GICs. The periods of high GIC pulses are generally of the most concern for power system impacts and operation. ac + dc ac Time ac Time figure 2. Half-cycle saturation. B I ac + dc GIC risk is highly dependent on actual characteristics of a GMD event and the parameters of each power grid. Factors that influence the degree of risk to grid and equipment include geomagnetic latitude local ground resistivity the network configuration, together with capabilities of key equipment. Effect of GICs on Power Transformers Basics of Effect of GICs on Transformers GICs flowing through high-voltage (HV) transformers is the root cause of nearly all GMD-related issues. It causes halfcycle saturation in power transformers, which can result in issues such as increased harmonic generation, transformer heating, and system voltage instability. When a power transformer is subjected to dc current in the windings, it results in a unidirectional shift of the core flux. DC flux adds to the ac flux in one half cycle, as shown in Figure 2. The amount of dc flux density shift depends on magnitude of the dc, number of turns in the windings, and the magnetic reluctance that the transformer provides to the dc flux. The large magnetizing current pulse, shown in red in Figure 2, increases the effective reactive power absorbed by the transformer. Therefore, the power network sees a large increase in var demand for the duration of the GIC flow. The transformer magnetizing current pulse also injects significant amounts of even and odd harmonics into the connected power system. Figure 3 depicts the relationship between the var loss (Q exc) and the GIC for a large single-phase power transformer of core form construction. These data are calculated using the fundamental frequency of both voltage and current. Typical Profile of GIC Pulses Figure 4 presents the signature of a GMD event that occurred during March The curve represents the absolute 72 ieee power & energy magazine july/august mpe4-pestaskforce indd 72 6/6/13 3:11 AM

3 A GMD event can last one to two days and continually generates relatively low to moderate levels of GICs with several intermittent periods of high GICs. value of GIC calculated from magnetic field measurements made at the Ottawa, Canada, geomagnetic observatory. For purposes of illustration, the GICs have been scaled to produce a 1-A per phase peak magnitude. GIC is a quasi-dc pattern with hours-long periods of low to medium magnitudes relative to its GIC peaks. This sustained activity appears as a series of spikes (rather than constant dc) because of the observation time frame. The peaks of GIC activity are separated by an hour or more for this event, and in terms of the transformer s thermal time constant each individual GIC peak can be considered as an isolated event. Figure 5 presents recorded transformer data during a different GMD event on 1 May It includes neutral GIC, the real and reactive power loss, and the third and sixth harmonic currents in a 345/115 kv autotransformer. Figure 5 shows a 12-min total duration portion of a GMD event that rises to about a 4-min duration peak of GIC of 8 A (or ~27 A per phase). During the peak, there is a significant jump in the transformer s reactive power demand as well as harmonic current injection as a result of the GIC flow in the transformer. Thermal Effects of GIC on Power Transformers During GIC flow, high magnitudes of magnetization current pulses and associated current harmonics produce increased, harmonic-rich stray flux. This results in much higher eddy and circulating current losses in the windings as well as in the structural parts of the transformer. The resultant increase in load losses and temperatures of windings and structural parts must be assessed individually for each power transformer design since the GIC-imposed thermal duty is outside standard service parameters. Expected thermal performance due to GICs can be represented by a simplified step signature, as shown in Figure 6. This simulates a thermally demanding 1-h period from the storm and consists of a base GIC level (i.e., sustained low-level GIC flow during the storm) and in this case a peak GIC magnitude with 6-min duration. Figure 7 presents the calculated winding and tie-plate hot-spot temperatures in a fully loaded, large A/Phase single-phase power transformer when subjected to the GIC profile in Figure 6. Excursions of winding hot-spot temperatures are shown in Figure 7(a) and are determined by the peak GIC magnitude that lasts only a few minutes. Thus, winding temperatures increase when high magnetization % MVA Rating GIC, A/Phase figure 3. An example of reactive power drawn by a large single-phase power transformer (in % of rated MVA). " Time (h) A/Phase Time (min) figure 4. Calculated GICs in a large power transformer neutral during a 48-h period. july/august 213 ieee power & energy magazine 73 11mpe4-pestaskforce indd 73 6/6/13 3:11 AM

4 Natural Current (A) currents flow, but such increases would not cause any winding damage or any significant loss of winding insulation life. Similarly, for the thermal effect of GICs on the structural parts, it is shown in Figure 7(b) that the maximum tie-plate temperature rise due to the peak GIC magnitude is minimal. Such maxima of structural part temperatures are acceptable for the expected short duration of peak GIC magnitudes. Depending on the transformer design and actual GIC magnitudes and duration, the tie-plate maximum temperatures could possibly reach temperatures that would produce small amounts of dissolved gas and would not have much consequence on the reliability of the transformer. Industry standards allow much higher structural parts maximum temperature levels for much longer times under emergency loading conditions (e.g., 3 min). A/Phase MW 3rd 6th 6 min 1 A GIC 6 min 65 A Mvar -1 9:25 9:3 9:35 9:4 1 May 1992 (Universal Time) 24 min 35 A 4 min 6 A 18 min 2 25 A 1 A 2 min Time (min) figure 6. A simulated GIC current profile. 9:45 9:5 figure 5. Neutral GIC, third and sixth harmonics, transformer real and reactive losses MW/Mvar The long-duration GIC profile shown in Figure 4 shows that a high GIC magnitude will exist for only a few minutes, and each such maximum is separated by relatively long times. The GIC current profile adopted in Figure 6, and the corresponding calculated hot-spot temperatures presented in Figure 7, illustrates that the GIC causes hot-spot temperature rises for only a few minutes per GIC maximum event. Finally, it is to be noted that the above maximum temperatures of windings and structural parts were calculated while the transformer is fully loaded and the ambient temperature is 3 o C. The calculated winding and structural parts temperatures will be lower when the transformer is not fully loaded and/or ambient temperature is lower. Another consequence of the unidirectional flux density shift in the core is that significant increases in both core losses and core noise are experienced for the duration of the GMD event. The loss increase in the core results in an increase in the core hot-spot temperature. However, the core s thermal stabilization time is typically much longer (3 45 min) than the duration of the high peaks of GIC and correspondingly produces smaller increases in the core temperatures. While core noise increase is typically very noticeable onsite and is associated with higher magnitudes of core and tank vibrations, the increase of the core noise is only temporary and is limited to the duration of the GMD event. GIC Capability of a Transformer Design The GIC capability of a transformer design is determined by the maximum allowed temperatures for the windings and structural parts due to combinations of load current and GIC current. For base GIC level (i.e., sustained low-level GIC flow during the storm), the temperature limits recommended by the loading guide from IEEE Standard C57.91, for long duration overloading of transformers, can be used. Correspondingly, the limits recommended by the same standard for short-duration emergency overloading can be used for the peak GIC magnitude GIC flows of short duration. The temperature limits shown in Table 1 can be used for time-dependent GIC thermal evaluation. As stated by IEEE Standard C57.91, the purpose of these recommended temperature maximums is to limit, to a reasonable value, the rate of loss of life of the solid insulation used in the transformer and also to prevent gas bubbles in the oil. Actual Measurements of Effect of DC on Temperatures of Windings and Structural Parts Two experiments were performed by Hydro Quebec and Fingrid in which large in-service, single- and three-phase power transformers were injected at no load with high levels of dc (75 A/phase for up to 1 h and up to 2/3 A/phase for 2 min ramped up in 3-min intervals, respectively). Temperature rises were measured in windings and structural parts of the transformers. When compared to the IEEE Standard C ieee power & energy magazine july/august mpe4-pestaskforce indd 74 6/6/13 3:11 AM

5 temperature limits, winding temperature rises were low and temperature rises in the structural parts were moderate. Both reports conclude that at the GIC levels injected, no transformer damage would occur. Reported Transformer Damage/ Overheating Attributed to GIC A typical analysis of GIC induced thermal events assumes thermal excursions caused only by GIC induced stray flux within the transformer. In the case of those power transformer designs where half-cycle saturation would cause high winding circulating currents, it is necessary to determine the possibility of winding overheating when the transformer is subjected to high levels of GIC. The significant overheating of the series connection of old (pre-1973) design shell form transformers at PSE&G during the 1989 GMD storm was an example of this group of transformers. Reported incidents of tank wall overheatings are a typical consequence to core saturation but should not impact the reliability of the transformer in any negative way. The high levels of localized tank overheating reported in one case was caused by limited oil flow between the core and the tank wall due to presence of the wooden planks inherent in the design of the older shell form unit. Temperature ( C) Temperature ( C) Effects of GIC on Power Systems Quasi-dc current flow in transformer windings is characterized by a significant increase in excitation current (see Figure 2) as a result of half-cycle saturation. This excitation current is nonsinusoidal and comprised of a large fundamental component with even and odd harmonics. The immediate consequences of GICs and half-cycle saturation are the generation of even and odd harmonic currents and an increase of reactive power absorbed by transformers. This can have a significant effect on var resources, var margins, generator performance, and protective relaying. Voltage Stability Unlike the traditional I 2 X loss associated with load current flow through the transformer, var loss associated with halfcycle saturation is a shunt loss, where the transformer behaves as a large reactive load. Figure 3 illustrates the relationship between the var loss (Qexc) and GIC for a large single-phase core form transformer. Half-cycle var loss increases proportionally to GIC flow in transformer windings and (even for moderate amounts of GIC flow); this var loss increase may reduce system voltages to the point of encroaching secure voltage limits. If var resources such as capacitor banks, static var compensators (SVCs), and spinning reserves are exhausted during a GMD event, a voltage collapse can occur Calculated Windings Hot-Spot 116 Temperature at Full Load Time (min) (a) Calculated Tie Plates Hot-Spot Temperature at Full Load Time (min) figure 7. Calculated (a) winding and (b) tie-plate hot spot temperatures using the GIC profile of Figure 6. (b) on a single contingency. If var supplies are near the point of exhaustion, extreme operating actions, such as curtailing load, could be necessary to provide mitigation. Thus, the reactive power loss, resulting from half-cycle saturation of transformers is one of the major concerns during GMDs. Harmonics Effects GIC-induced harmonic currents generated by saturated transformers have a significant systems impact. Shunt capacitor banks used for var support become low impedance paths for harmonic currents and can lead to tripping of the bank by relay protection schemes. Harmonic filters for SVCs create table 1. Temperature limits. Base GIC Cellulose insulation 14 o C 16 o C Structural parts 16 o C 18 o C Short-Duration GIC Events july/august 213 ieee power & energy magazine 75 11mpe4-pestaskforce indd 75 6/6/13 3:12 AM

6 Geomagnetic disturbances are naturally occurring phenomena that produce geomagnetically induced current in transmission lines of the modern power grid. parallel resonances that, if located at characteristic harmonic frequencies, can exacerbate voltage distortion issues and result in increased harmonics flow in these devices and tripping on protection. Harmonics can also cause the misoperation of electromechanical and solid-state relays, resulting in either nuisance operations or failure to operate when required. In the case of modern digital relays, where harmonic currents may be filtered, overcurrent protection of capacitor banks would become desensitized, thus reducing its effectiveness and potentially leading to capacitor bank damage. Generator Effects Generators are not exposed to the flow of GICs directly because GIC flow is confined to the grounded return path of the HV winding of the generator step-up (GSU) transformers. However, generators are susceptible to the effects of harmonics and unbalanced voltages that are caused by the flow of GIC in the GSU itself and other system transformers. When harmonic current is injected into the stator winding of a generator, harmonic currents are induced into the rotor circuit. This harmonic rotor current can cause heating of the rotor surface and possible arcing damage of rotor slot wedges. Conventional negative sequence relays may respond improperly, or not at all, to the flow of harmonic currents. Thus, conventional relaying may not provide adequate protection to the generator rotor during a GMD. Although no serious generator damage due to GMDs has been documented, analysis indicates the potential for damage or unit trips. Notification of Impending GMD events The Space Weather Prediction Center (SWPC) of the National Oceanic and Atmospheric Administration (NOAA) has developed a three-day space weather forecast to improve understanding of space weather events among power grid system operators, asset owners, satellite/communications owners, and the general public. While the forecast predicts potential solar storm activity, it does not predict the likely effect on technological systems. For geomagnetic storms, K and G scales estimate the probability of occurrence and magnitude for each of the next three days. Space weather alerts for potential GMDs are issued to regional transmission operators (RTOs) and transmission planners when observatory magnetometer readings show nano-tesla fluctuation levels in excess of the minimum level of potentially detrimental GMD activity. A number of RTOs have GIC monitors located in coastal substations and other extra HV substations deemed susceptible to GICs. Many GIC monitors not only allow for neutral dc detection but also provide harmonic information, especially the second harmonic that shows a strong correlation with GIC. Mitigation of the Effects of GIC After receiving an alert from NOAA and verifying GIC in the area, the RTOs and/or the local utility can put operating procedures or individual equipment protection into effect. Different procedures can be implemented depending on the lead time of the triggers (e.g., NOAA alert/watch/warning versus observation of actual equipment/system effects). Increased Situational Awareness Monitoring and assessment procedures can prepare system operators for responses unique to a GMD event. If GIC flows occur, these steps are necessary to provide indication of system stress. Monitor unusual voltage and/or var swings. Monitor abnormal temperature rise/noise/dissolved gas in transformers. This requires built-in instrumentation. Prepare for unplanned capacitor bank/svc tripping. Monitor GICs on transformer banks equipped with monitors some jurisdictions use 1 A per phase GIC as a trigger for additional situational awareness when actual limits are unknown for vulnerable transformers, GIC versus time withstand capability curves can be determined via OEM analytical studies real-time mitigation of transformer effects (after a storm is already in progress) cannot be based solely on a single indicator (e.g., increased GIC); one or more additional indicators should be monitored to determine adverse transformer and system effects (e.g., increased Mvar loss, abnormal temperature rise, harmonics, etc.) reactive power loss and harmonic distortion information acquired through system control and data acquisition (SCADA). Monitor Mvar loss of all extra high-voltage transformers where possible. Monitor reactive power reserves. Prepare for possible false energy management system indications if telecommunications systems are disrupted (e.g., over microwave paths). Prepare for possibly inaccurate indications from state estimators that do not take into account var loss due to transformer half-cycle saturation. 76 ieee power & energy magazine july/august mpe4-pestaskforce indd 76 6/6/13 3:12 AM

7 Safe System Posturing These procedures increase system operating margins and allow equipment to tolerate increased reactive/harmonic loading; reduce transformer operating temperature, allowing additional margins in the temperature rise from core saturation; and help prepare for contingencies such as the possible loss of transmission capacity due to the loss of a transformer. If possible, return out-of-service equipment to operation. Delay planned outages. Start offline generation, and synchronous condensers. Redispatch generation (possibly implement autorunback if available). Manually start fans/pumps, where possible, on selected transformers to increase thermal margins. Observe conservative operation modes with possibly reduced transfer limits. System Reconfiguration These options should be implemented only if supported by study. Before opening breakers, the effect of dc bias on breaker operation should be considered, noting that it is only a concern if the GIC flow exceeds the peak ac current flow. Remove transformer(s) from service if there is imminent damage due to overheating. Remove transmission line(s) from service to redirect GIC flow. Note that removing transformers from service may increase the GIC effects on neighboring equipment, especially where there are multiple transformers in a given station. Stations with autotransformers may behave in a counterintuitive way (due to GIC flow-through and cancellation effects) that can only be assessed by study. In some cases, the guidelines for protecting equipment may conflict with the guidelines for secure system operation. For instance, guidelines that remove equipment from service or reduce generator loading may increase loading on other equipment, putting the system in a less secure state. Operators may be required to evaluate these trade-offs with very limited information, thus the importance of carrying out studies ahead of time. Deciding when to return to normal operation can be difficult. The intermittent nature of the effects of geomagnetic storms makes it difficult to tell when the storm activity is over. There may be lulls in activity followed by additional severe activity. Many utilities return to normal operation generally 2 4 h after the last observed geomagnetic activity. GIC Blocking Devices One possible means to eliminate, or reduce, GICs from entering the transformer through the grounded neutral is by inserting a resistor or a capacitor from neutral to ground. However, in autotransformers, GIC blocking devices connected in the neutral path do not prevent the flow of GIC through the series winding and therefore do not prevent core saturation. There is the potential for unintended consequences if blocking devices are employed, and a number of effects must be considered on a case-by-case basis. 1) Close-in line-to-ground faults can generate transient overvoltages that can cause surge protection devices associated with the blocking device to fail. 2) Transformer neutral insulation coordination must be evaluated to ensure proper margins are maintained during faults. 3) Series resonance and ferroresonance can also occur and must be evaluated. 4) Line-to-ground and double line-to-ground fault currents increase due to the impedance cancellation effect of capacitive blocking devices on the zero sequence impedance. 5) Redirection of GIC to other transformers. Transformer manufacturers may also have to address issues related to warranty enforcement or if there are special requirements for neutral insulation coordination when the customer indicates that a blocking device will be installed on the transformer neutral. Industry Efforts to Analyze GIC Early industry efforts to increase the understanding of GICs have produced a fair amount of independent research but on a relatively small scale, as past GIC experience has only occasionally resulted in any significant system issues. There are a number of ongoing research, analysis, and testing efforts that include academia, utilities, government, equipment manufacturers, and research institutions. The knowledge base on the effects of GIC must be expanded to produce a system of GIC mitigation best practices. Measurement GICs are typically measured at the grounded neutrals of power transformers. Until recently, devices to measure dc were custom built but several manufacturers have added modules and sensors to transformer electronic monitoring devices. These devices measure and log dc neutral current along with other monitored online parameters, such as temperature and dissolved gasses. In addition, some sensors monitor even harmonics (such as second, fourth, etc.) that appear during half-cycle saturation. A GIC measurement by proxy uses digital relay harmonic output from transformer differential protection. A number of utilities are considering this approach, since harmonic distortion and var loss are clear indicators of GIC occurrence in system operation. Modeling To assess voltage stability, system modeling and analysis software must represent GIC flow, transformers in saturation, and related reactive power losses. Several off-the-shelf software programs now include GIC calculation capabilities, and EPRI has an open-source GIC software tool. These tools rely on a number of modeling assumptions with limited july/august 213 ieee power & energy magazine 77 11mpe4-pestaskforce indd 77 6/6/13 3:12 AM

8 This article also highlights the issues of voltage instability and the threat that malfunctioning of relay protection schemes are possible due to GIC-induced harmonics. validation, so measured data such as dc currents, var loss, and harmonics in the presence of GMD events are essential to the validation of analytical tools and their improvement. Local geological variations and geomagnetic latitude are vital to the correct representation of geoelectric field magnitude and GIC flow. Correct representation of field intensity is not derived from existing system planning software but rather will require input from geophysicists. A number of initiatives are under way to estimate maximum and typical storm scenarios that can be used for power system analysis. Summary GMDs are naturally occurring phenomena that produce GIC in transmission lines of the modern power grid. They will continue to occur and will have an effect on power system operation and reliability as transmission systems becomes more interconnected and ever more complex. The power industry must continue to pursue the understanding of GICs to be able to accurately predict the effects that will occur on the power systems so the owners and operators will be able to implement adequate and prudent mitigation actions should the need arise. Measurement, modeling, and mitigation implementation cannot be performed independently as the effects are too widespread. Due to the interconnection of the electrical grid, a coordinated effort is necessary to provide the industry with a set of options for implementation at a variety of locations with varying degrees of risk. This must be suitable and prudent while verifying that solutions employed in one region will not adversely affect another location. The management of the effects of a GMD event must lie within the realm of science and solid engineering. This article has focused on the effects of GIC on the power transformers used across the grid. While GMD events will subject transformers to higher thermal and, to a lesser extent, mechanical stress, peer reviewed analyses performed thus far indicate that most transformers will see no damage from an extreme GMD event. This article also highlighted the issues of voltage instability and the threat that malfunctioning of relay protection schemes are possible due to GIC-induced harmonics. There are a number of mitigation techniques, including those mentioned in this article, that address these possibilities. Additional efforts are necessary to understand GMD and its impact on the bulk power system and the optimization of mitigating measures. The IEEE Power & Energy Society is committed to support such efforts. Finally, planners and operators must have the technical tools to perform vulnerability assessments and develop mitigating solutions, as necessary. The development of these tools is ongoing and includes a combination of a) developing tools to perform integrated GIC and power flow analysis for a variety of system conditions, configurations, and contingencies b) developing geoelectric fields representative of a design basis GMD event (e.g., a one in 1-year storm) for a variety of latitudes and ground conductivity structures c) developing accurate thermal models for power system equipment suitable for implementation in power system planning tools. Establishing a set of guidelines to mitigate the potential harm done by GIC is a complex but manageable task that is best accomplished by improved modeling, solid engineering principles, and the cooperative efforts of all stakeholders. For Further Reading R. L. Lesher, J. W. Porter, and R. T. Byerly, SUNBURST: A network of GIC monitoring systems, IEEE Trans. Power Delivery, vol. 9, no. 1, pp , Jan R. Girgis and K. Vedante, Effect of GIC on power transformers and power systems, presented at the IEEE T&D Conf., Orlando, FL, May 212. Solar magnetic disturbances/geomagnetically induced current and protective relaying, Electric Power Research Institute, Palo Alto, CA, Rep. TR-12621, Aug W. B. Gish, W. E. Feero, and G. D. Rockefeller, Rotor heating effects from geomagnetic induced currents, IEEE Trans. Power Delivery, vol. 9, no. 2, pp , Apr Effects of geomagnetic disturbances on the bulk power system, NERC 212 Special Reliability Assessment, NERC GMD Task Force, North American Electric Reliability Corporation, Atlanta, GA Interim Rep., Feb IEEE/PES Technical Report on Geomagnetic Disturbance and Its Impacts on the Power Grids, IEEE Power & Energy Society Technical Council Geomagnetic Disturbance Task Force, Piscataway, NJ, May 213. Biography This article is the result of a collaborative effort led by the IEEE Power & Energy Society Technical Council Task Force on Geomagnetic Disturbances that comprised of industryrecognized experts from IEEE Power & Energy Society technical committees such as the Transformers Committee, the Power System Relaying Committee, and the Power System Operating Committee. p&e 78 ieee power & energy magazine july/august mpe4-pestaskforce indd 78 6/6/13 3:12 AM