By Sarina Adhikari

The Federal Energy Regulatory Commission (FERC) had directed North American Electric Reliability Corporation (NERC) to develop standards related to system operating during Geomagnetic Disturbance (GMD) and system impact analysis to predict possible GMD vulnerability. Both the NERC GMD operating standard, EOP-010-1 and the GMD planning standard, TPL-007-1 have already been accepted and enacted by FERC. The standard, TPL-007-1 requires transmission planners to perform detailed assessments of GMD vulnerability, based on a prescribed level of GMD intensity.

During geomagnetic disturbance (GMD) events large, slowly-pulsating auroral electrojets in the earth’s magnetosphere induce very low-frequency voltages in series with power transmission lines. Grounded-wye transformers provide a path for the circulation of induced Geomagnetically Induced Currents (GIC). The flow of GIC through transformers results in potentially severe asymmetric (half-cycle) saturation. Transformer saturation can impose potentially damaging thermal duty on transformers. Each saturated transformer acts as a reactive sink; the aggregate of many saturated transformers can cause a large reactive power deficit, possibly resulting in system voltage collapse.  Transformers saturated by GIC inject harmonic currents that have even greater magnitude than the fundamental-frequency reactive current they absorb. The resulting harmonic voltage distortion caused by simultaneous harmonic currents injected by transformers throughout the system can:

  • Overload capacitor banks;
  • Overload the underground transmission cables; and
  • Overheat generator rotors

Protective tripping of these devices can deprive the system of critical reactive support at a time when the system is highly stressed from the excessive reactive power demand, potentially hastening the onset of voltage collapse. 

Therefore, the GMD vulnerability assessment studies are very important for safe, secure and reliable grid. 

EnerNex LLC partnering with Walling Energy Systems Consulting (WESC) recently completed a very comprehensive GMD study for the United Illuminating Company (UI) that is located in the southwest area of Connecticut along the Long Island Sound. UI’s system is located in a relatively high geomagnetic latitude (latitude relative to the north magnetic pole, which is located in northern Canada), in an area where the geologic structure favors higher GMD intensity, and is on a coastline; all factors tending to increase GMD vulnerability. The GMD study area considered for this study comprised of not only the UI service territory but also included the entirety of Connecticut, some part of Rhode Island and western Massachusetts. The GIC flow and power flow model were developed in GE PSLF GMD module. Harmonic analysis was performed in NexHarm Software.

Important Contributions:

  • This is a groundbreaking and realistic study of GMD impacts on a transmission system that included GIC flow, fundamental-frequency power flow and harmonic analyses with a novel integration of diverse software tools
  • The GIC model considered all the important GMD model data such as longitude, latitude, substation grounding resistance, transformer types, etc., precisely for the entire study area
  • The conventional practice of assuming reactive demand of a GIC saturated transformer being directly proportional to the net GIC and the reactive current being voltage independent applies only for single phase transformers
  • Three phase transformers have relatively non-linear reactive current Vs GIC characteristics and the reactive currents of these transformers are also very sensitive to bus voltage magnitude
  • Above mentioned conventional assumptions for three phase transformers lead to an excessively pessimistic reactive demand during GMD events
  • Therefore, in this study:
    • Tables defining the reactive current as a function of GIC and voltage level were developed using detailed magnetic circuit modeling for different types of transformers
    • These tables are utilized by PSLF GMD package while calculating realistic GIC flows
    • The same magnetic duality models were used to obtain harmonic current magnitude and phase angle for various transformer types

Significant Findings:

  • GIC model is more accurate if a 115 kV system data is also included in the model. NERC’s GMD planning standard, TPL-007-1 suggests modeling the system only above 200 kV
  • System collapse is not expected to occur for GMD up to the standard field intensities defined by NERC TPL-007-1, but could occur for higher intensities
  • Harmonic resonances driven by GIC saturation of numerous transformers can:
    • result in very high levels of voltage distortions that interfere with protection systems
    • cause capacitor banks to trip making the system prone to voltage collapse
  • System voltage collapse may occur at GMD intensities less than that are likely to cause direct risk to transformers
  • Harmonic impacts may be disruptive at lower GMD intensity than the threshold of voltage collapse

A poster containing some results from this GMD system impact study can be found here.

For more information, contact us.