TRANSMISSION LINE SWITCHING OPERATIONS

Surges associated with switching transmission lines (overhead, SF6, or cable) include those that are generated by line energizing, reclosing (three phase and single phase operations), fault initiation, line dropping (de-energizing), fault clearing, etc. During an energizing operation, for example, closing a circuit breaker at the instant of crest system voltage results in a 1 pu surge traveling down the transmission line and being reflected at the remote, open terminal. The reflection interacts with the incoming wave on the phase under consideration as well as with the traveling waves on adjacent phases. At the same time, the waves are being attenuated and modified by losses. Consequently, it is difficult to accurately predict the resultant wave-shapes without employing sophisticated simulation tools such as a transient network analyzer (TNA) or digital programs such as the Electromagnetic Transients Program (EMTP).

Transmission line over-voltages can also be influenced by the presence of other equipment connected to the transmission line; shunt reactors, series or shunt capacitors, static VAR systems, surge arresters, etc. These devices interact with the traveling waves on the line in ways that can either reduce or increase the severity of the over-voltages being generated.

When considering transmission line switching operations, it can be important to distinguish between “energizing” and “reclosing” operations, and the distinction is made on the basis of whether the line’s inherent capacitance retains a trapped charge at the time of line closing (reclosing operation) or whether no trapped charge exists (an energizing operation). The distinction is important as the magnitude of the switching surge overvoltage can be considerably higher when a trapped charge is present; with higher magnitudes, insulation is exposed to increased stress, and devices such as surge arresters will, by necessity, absorb more energy when limiting the higher magnitudes. Two forms of trapped charges can exist; DC and oscillating. A trapped charge on a line with no other equipment attached to the line exists as a DC trapped charge, and the charge can persist for some minutes before dissipating.

However, if a transformer (power or wound potential transformer) is connected to the line, the charge will decay rapidly (usually in less than 0.5 sec) by discharging through the saturating branch of the transformer. If a shunt reactor is connected to the line, the trapped charge takes on an oscillatory wave-shape due to the interaction between the line capacitance and the reactor inductance.

This form of trapped charge decays relatively rapidly depending on the Q of the reactor, with the charge being reduced by as much as 50% within 0.5 seconds.

FIGURE 10.21 DC trapped charge.

FIGURE 10.22 Oscillating trapped charge.

The power system configuration behind the switch or circuit breaker used to energize or reclose the transmission line also affects the over voltage characteristics (shape and magnitude) as the traveling wave interactions occurring at the junction of the transmission line and the system (i.e., at the circuit breaker) as well as reflections and interactions with equipment out in the system are important. In general, a stronger system (higher short circuit level) results in somewhat lower surge magnitudes than a weaker system, although there are exceptions. Consequently, when performing simulations to predict over-voltages, it is usually important to examine a variety of system configurations (e.g., a line out of service or contingencies) that might be possible and credible.

Single phase switching as well as three phase switching operations may also need to be considered. On EHV transmission lines, for example, most faults (approximately 90%) are single phase in nature, and opening and reclosing only the faulted phase rather than all three phases, reduces system stresses. Typically, the over-voltages associated with single phase switching have a lower magnitude than those that occur with three phase switching. Switching surge overvoltages produced by line switching are statistical in nature; that is, due to the way that circuit breaker poles randomly close (excluding specially modified switchgear designed to close on or near voltage zero), the instant of electrical closing may occur at the crest of the system voltage, at voltage zero, or somewhere in between. Consequently, the magnitude of the switching surge varies with each switching event. For a given system configuration and switching operation, the surge voltage magnitude at the open end of the transmission line might be 1.2 pu for one closing event and 2.8 pu for the next and this statistical variation can have a significantly impact on insulation design.

FIGURE 10.23 Phase-to-ground overvoltage distribution.