C H A P T E R
N ° 15
Space Weather and Power Grids
The type of critical infrastructure that society has become the most reliant on is the power grid. Power networks play a vital role in everyday life. It can be a standalone critical infrastructure providing electrical power but it can additionally be a service provider to other critical infrastructures that rely on electricity from a power grid.
The energy sector has been aware of the impact of space weather on power grid systems for years. Many studies have shown that space weather can affect power systems through geomagnetically induced currents (GIC). Geomagnetically induced currents (GIC) are exceedingly low frequency field (i.e., quasi-DC) currents which can induce electric currents that can get into the power transmission lines and flow to and from the ground through the windings of power transformers. These currents can cause numerous issues when entering a power grid.
In today’s article, we will look closer at the main damage and failure modes associated with Geomagnetically Induced Currents (GIC) loading in power grids.
Image Credit: Xuejian Zhang et al. (2024): Illustration showing geomagnetically induced currents (GIC).
Transformer saturation
Transformers are a vital part of the power grid, as they are used to increase voltage levels for electricity transport in transmission lines, and to reduce the voltage for electricity distribution to costumers. They use steel cores and are designed to be extremely efficient, and they usually operate in the linear range of their magnetic characteristic. This corresponds to an exciting current of only a few Amperes of Alternating Current (AC). Alternating Current (AC) is a type of electrical current, in which the direction of the flow of the electrons switches back and forth at regular intervals or cycles, and is used in power lines and household wall outlets.
Transformers have shown to be particularly susceptible to the impact from geomagnetically induced currents (GIC), as they are not designed to handle the Direct Currents (DC) (i.e., a one-directional flow of electric charge). According to studies conducted in 2010 and 2012 by scientists such as Triomphant Ngnegueu and John Kappenman, geomagnetically induced currents (GIC) flowing into the transformer system can cause the operating point on the steel core saturation curve to shift from the linear relation between input and output voltages and currents towards the non-linear region, leading to cascading effects (i.e., secondary effects).
This consequently creates saturation within one half of the cycle, causing a very high and asymmetrical exciting current to be drawn by the transformer (~10-15% or more of the rated load current). However, different types of transformers are impacted differently by these half-cycle saturations. Single-phase transformers have shown to be more vulnerable than three-phase transformers. This is because the quasi-Direct Current (DC) flux induced by the geomagnetically induced currents (GIC) can flow directly in the core. Furthermore, shell-type transformers have shown to be more vulnerable than core-type transformers, while autotransformers are particularly susceptible.
Image Credit: Atlearner: Illustration of Direct Current (DC) and Alternating Current (AC).
According to a paper published in 2002 by the electrical engineer Tom S. Molinski, almost all issues with power grid equipment and operations caused by space weather arise from disturbed transformer performance, which is forced into half-cycle saturation by the geomagnetically induced currents (GIC). The cascading effects could be an increase in reactive power consumption and the injection of even and odd harmonics into the power system. These odd or even harmonics can cause even less compensating reactive power to be available, eventually leading to a collapse of the power grid. Furthermore, according to electrical engineer David H. Boteler, the greatest impact is to be found in long transmission lines (e.g., +100 km long), as longer lines have higher voltage support requirements.
Without transformers, electricity generation and transmission over long distances would not be possible nor would it power urban centers and industrial complexes.
Transformer overheating
Image Credit: Jørn Foros; SINTEFblog: Picture of transformer.
In case of transformer saturation, most of the excess magnetic flux flows externally to the core into the transformer tank. Here, currents are created, and localized tank wall heating with temperatures reaching 175°C can occur.
Repeatedly exposure to heating caused by geomagnetically induced currents (GIC) loading can lead to cumulative insulation damage, accelerated ageing, and eventually transformer failure. In this case, it can be difficult to relate cause and effect. Thus, the transformer failure could be attributed to other reasons, as operators will not experience the effect of the geomagnetically induced currents (GIC) immediately.
Reactive power loses
In contrast to active power which is consumed by the load, reactive power is power that is reflected back to the grid. Similar to the pressure that pushes water through a pipe, voltage acts as the pressure that pushes electrical current through power lines. To do this, voltage draws on reactive power. Single-phase transforms consume the largest amount of reactive power.
Transformers saturated by geomagnetically induced currents (GIC) loading have a higher reactive power consumption. This increases linearly with the magnitude of the geomagnetically induced currents (GIC). A 90-degree voltage shift caused by the excitation current during saturation creates a reactive power demand from the power system. As a consequence, there may be drops in system voltage and the stability margins may decrease significantly because additional reactive power is being consumed. If voltage support devices trip during the geomagnetically induced currents (GIC) event, the situation would be exacerbated because of the injection of harmonics into the system.
Harmonics
When a transformer is driven into half-cycle saturation, the exciting currents contain harmonics of various orders (fundamental, 2nd, 3rd, etc.) giving rise to complex current patterns. In cases of very large geomagnetically induced currents (GIC) levels, the contribution of harmonics declines, especially at the higher orders, since the transformer is operating in a completely linear – although saturated – region of its magnetizing curve.
Power grids are generally designed to cope with odd harmonics. However, they can be overwhelmed by even harmonics, like negative sequence 2nd harmonic. This is because they usually are not expected during power operations. A consequence of this can be false neutral overcurrent relay actuation. Furthermore, harmonic currents can cause series losses in things such as circuit breakers and filter banks.
Generator overheating
Usually, generators are shielded from direct impact from geomagnetically induced currents (GIC). However, they can still be affected by harmonics and voltage unbalance caused by transformer saturation caused by geomagnetically induced currents (GIC). If harmonic currents enter the generator, it increases the risk of excessive heating and mechanical vibrations. As the higher harmonic orders are concentrated near the rotor surface, this can heat up and create cracks. The consequences of such damage may not appear immediately, yet they may be experienced at a later time/stage as for example failures in the system. As the consequences may be experienced at another time, it may end up not being attributed to impact from geomagnetically induced currents (GIC).
Protection relay tripping
A protective relay is a compact and self-contained switchgear that trips a circuit breaker when a fault is detected. This can be for conditions such as overcurrent, overvoltage, over- and under-frequency, and reverse power flow. If the current rises above a certain limit over a certain period of time, the overload relay will trip, operating a secondary contact which interrupts the motor control circuit. This deenergizes the contactor, leading to the removal of the power to the motor.
Modern digital relays measure the peak current value to monitor the status of the system. They are sensitive to tripping by harmonics. During geomagnetically induced currents (GIC) flows the harmonic content of the power system increases. This causes false trips that can potentially trigger a cascading failure of the power system. The relay’s set current can be adjusted to accommodate the higher harmonics during geomagnetically induced currents (GIC) and, thus, reduce the risk of false trips. However, this comes at a cost of lower protection levels.
Power systems increasingly depend on reactive power compensators and shunt capacitor banks for voltage control. Shun capacitor banks are primarily used to improve the power factor in the network. Additionally, they improve the voltage stability and reduce network losses. By improving the power factor, it provides the capability of providing higher power transmission and increased control of the power flow. Generally, shunt capacitors are grounded and have protection against unbalanced operation through neutral overcurrent relays. However, these capacitor banks are vulnerable to false trips during geomagnetically induced currents (GIC) because of the capacitor’s low resistance at the associated harmonic frequencies.
According to The Institute of Electrical and Electronics Engineers Inc., many power grid operators have upgraded or replaced their neutral overcurrent unbalance protection to reduce the likelihood of false trips.
Effects on communication systems for power grids
Space weather causing geomagnetic storms affect long-haul telephone lines, including undersea connections and internet cables. Long-haul networks are designed to transmit data quickly and reliably using high-capacity long haul fiber optic cables, long-haul transceivers, and advanced networking technologies. The communication infrastructure directly used for power system operators and control is expected to be relatively unaffected, as the high bandwidth lines in Supervisory Control and Data Acquisition (SCADA) networks (used for controlling, monitoring, and analyzing industrial devices and processes) consists increasingly of optical fibers. These do not suffer the effect of electromagnetic disturbances. Yet, its weakness lies in its dependency on the services provided by the Global Navigations Satellite System (GNSS) such as the Global Positioning system (GPS) to obtain time stamps for things like phasor storm. Impact to something like this can cause severe temporary service degradation.
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