Geomagnetically induced current impact on other power system equipment

Other power system equipment with magnetic core may also be influenced by GIC flow over its windings. The detailed analysis of GIC impact on shunt reactors operation is represented in [52]. Shunt reactor’s sensitivity to GIC effects is rather low.

Due to its construction features, a relatively high GIC is required to cause shunt rector saturation. The current level is far beyond the maximum specified level. In power transformers, gaps have to be minimized in order to ensure efficient voltage and power transformation between the windings. Contrary to power transformers, shunt reactors contain wider non-magnetic gaps between core steel packets. Circuit breakers also contain magnetic core, but air gaps between core-steel packets are wider than in shunt reactors. Thereby, it is assumed that their susceptibility to GMD is admissible.

Modern power grids are characterized by high penetration of high voltage direct current (HVDC) convertors. HVDC convertors bring no barrier to GIC flow over its elements. The GIC flow by itself has no negative impact on HVDC convertor operation, since possible GIC amplitudes are negligibly small compared to nominal currents (kAs). The danger to HVDC converter might be posed from the side of abnormal high harmonics distribution due to power transformer saturation. The HVDC converters by themselves generate the current harmonics on the AC-side. Therefore, the means for limiting the current distortion to an acceptable level are already predesigned.

The orders of the generated harmonics are determined by the pulse number of the converter configuration defined as the number of simultaneous commutations per cycle of the nominal frequency. The modern HVDC systems consist of a 12- pulse converter, formed by connecting two 6-pulse bridges. One group has a wye - wye connected converter transformer, and the other group has a wye - Д connected converter transformer. It is convenient to classify the HVDC converter harmonics as characteristic and non-characteristic harmonics. The characteristic harmonics are those with the order pk ± 1, where к is any positive integer; p is a pulse number. The current on the secondary side /'2 can represented as the function of firing angle a and commutation angle у (Eq. 4.23):

The amplitude of the nth harmonic is given by Eq. 4.24:

The harmonics caused by imperfect system conditions are called noncharacteristic. In other words, the static converters generate harmonic orders and magnitudes not predicted by the Fourier series of the idealized waveforms. The main causes are listed in [53]. The compensation of non-characteristic harmonics is made by installing tuned filters. The single tuned filter is a serial RLC circuit that is tuned at one harmonic frequency. Its impedance is described in Eq. 4.25:

The series capacitors are also affected by high harmonic distortion. They perform as low-impedance paths for harmonics. Series capacitor’s impedance decreases with the frequency growth leads to the growth of currents amplitude. This can result in over-voltages. Series capacitors connected using the scheme “star with isolated neutral” have higher robustness to GMDs, since harmonics with the order 3к are blocked.

The transmission line operation conditions are predominantly limited by the admissible thermal forces which are chosen in accordance with its sag. The performed investigation showed that a GIC equal to 2 kA or higher can lead to excessing admissible values for transmission line sag. This current level is also far beyond the maximum specified level. The other problem is the non-uniform distribution of current into the conductor section. In other words, current will tend to flow closer to the surface of the conductor, which leads to uneven heat distribution in the conductor. The power of the flux of electromagnetic energy penetrating into the conductor through its surface and emerging in the conductor in the form of heat P can be calculated as in Eq. 4.26:

where / is the conductor’s surface through which electromagnetic wave penetrates; u is the perimeter of the conductor’s section; со is the current’s frequency; ц is the magnetic permeability; and у is the propagation coefficient.

The relative comparison of power system equipment susceptibility to direct and indirect GMD effects is presented in Table 4.8. The robustness of each equipment type to the GIC impact is represented in the first column. The level of impact on system operation in case of unit loss is shown in the second column and repair cost is given in the third column. Repair cost includes also replacement cost in case an equipment unit cannot be repaired swiftly.

Table 4.8

Comparison of power system equipment susceptibility to geomagnetically induced currents