Surge Arrester Stresses Due to Harmonic Resonance TOVs in Transmission Systems

Arresters

In general, the main electrical characteristics of underground and submarine cable circuits are significantly different to those of an overhead line circuit. One of these differences is that cable capacitance per unit length is 20-30 times higher than that per unit length of overhead line. As a result, when a new cable circuit is introduced into a grid or whenever an existing overhead line is replaced by cables, new resonance frequencies are created in the system’s harmonic impedance profile. These parallel resonances are ‘seen’ at the connections points and also at other points in the grid due to harmonic propagation.

Resonance frequencies depend greatly on system strength and their amplitude is defined by the system damping. Under contingency conditions, i.e. weaker system strength, the resonances are shifted to lower frequencies, which could be close to or at a low harmonic (2nd or 3rd). Fig. 1, for example, shows the harmonic impedance, as calculated for a particular node under intact as well as under system contingency conditions.

Fig. 1: Example of harmonic impedance profile.

The harmonic resonances can be excited under system transient conditions, thereby giving rise to temporary overvoltages and studies have reported that system events such as switching of power tranformers, e.g. scheduled energization and fault clearance, are regarded among the most onerous. The transformer inrush currents are rich in 2nd and 3rd harmonic components, which form the excitation source of the low frequency resonances.

These TOVs can dielectrically and thermally stress system components, causing them to age, deteriorate or even fail. According to international standards, components are tested against the standard power frequency TOV waveshape, which is based on a voltage shape of constant amplitude. Test voltage deviates significantly from harmonic resonance TOVs, which are characterized by an amplitude varying in time as well as by the harmonic content.

In addition, existing standards related to insulation coordination application guidelines do not provide methods to evaluate this kind of overvoltage. Rather, they recommend that TOVs due to harmonic resonance conditions should either be avoided or sufficiently suppressed by applying mitigation measures.

Although internationally accepted withstand levels and assessment methods are still not available, in 1998 CIGRE considered the TOV withstand capability of various components and published generic power frequency voltage-duration curves (see Fig. 2). At present and given that harmonic resonance TOVs are becoming a challenge for many utilities, Study Committee C4 Working Group C4.46 is preparing a Technical Brochure on “Evaluation of Temporary Overvoltages due to Harmonic Resonances in Power Systems”. The aim of this Working Group has been: 1) to provide suggested TOV assessment methods; and 2) to summarize common practices and modeling guidelines for performing electromagnetic transient (EMT) studies related to harmonic resonance TOVs

Fig. 2: Generic power frequency TOV voltage-duration curves.

Plan to attend the 2022 INMR WORLD CONGRESS in Berlin where expert Kostas Velitsikakis with TenneT in the Netherlands will make a presentation on surge arrester risks associated with harmonic resonance TOVs and also on their assessment through EMT simulations. He will also review the findings of a relevant case study involving the 400 kV Dutch transmission system.