A high voltage bushing is used to carry the current at high potential through an earthed wall or transformer/reactor tank. High voltage bushings of the capacitance graded type consist of three primary components: an outer insulation for minimizing creepage currents and preventing external flashover; an inner insulation (capacitance graded core, main insulation) for distributing the electrical field; and a conductor system for carrying the current. In the inner insulation there are several very precisely positioned, coaxial layers of conducting material in an insulating web.
The grounding of a bushing’s main insulation is done either by connecting the outmost conducting layer to the mounting flange, or to an external tap. Bushings designed according to IEC 60137 or IEEE C57.19.00 have either a test tap or voltage tap for grounding. For a test tap the outmost conducting layer of the main insulation (C1) is connected to the test tap and the capacitance between the outmost conducting layer and flange (C2) is grounded during service. For a voltage tap, the second outmost conducting layer of the main insulation is connected to the voltage tap forming C1. The outmost conducting layer in this case is a so-called C2 layer which is permanently connected to the grounded mounting flange, and thus the capacitance C2 is between the second outmost conducting layer and the outmost conducting layer. In this case there is also a C3 capacitance between the outmost conducting layer and the mounting flange.
During service, it is of utmost importance that the main insulation is grounded. This can be done in several ways, either via the lid when closing the tap, a grounding spring inside the tap or permanently connecting the main insulation to the grounded mounting flange (see left panel of Fig. 1). The latter is the most robust solution but eliminates the possibility of condition assessment of the main insulation.
In service, bushings can be exposed to transients, very fast transients (VFTs) or other high-frequency events. The transient voltages are created mostly from the switching operations of breakers especially in gas-insulated switchgear (GIS) applications. With the introduction of more distributed renewable generation (DERs) in the grid, more sources of harmonics and transients are introduced. The DERs are varying in nature with rapidly changing load profile as well frequent energization/de-energizations. VFT and harmonics are also increased with the introduction of more power electronics in the grid.
For safe operation of bushings in those conditions, it is important to have a low impedance between the main insulation and ground. It has been shown in several earlier studies that high impedance to ground may be the cause for bushing failures when exposed to high-frequency events. In cases with failures due to high impedance to ground, these most often occur in the tap region, cables connected to the tap or between the outmost conductive layer in the main insulation and ground. Internal high voltages between a bushing’s outermost conductive layer and the mounting flange originate from two different transient behaviors.
One is when the external system generates a high frequency oscillation with a frequency close to resonance frequency of the test tap inductance and the outmost layer capacitance to the flange. The other is due to the fast rise time of the transient voltage, the voltage over C2 will be a capacitive voltage division proportional to the transient step, due to that the high impedance to ground gives a longer time constant to discharge the capacitive circuit. Rise times in the order of a few nanoseconds has been measured from SF6 breakers close to a bushing. Both problems cause a breakdown in the insulation system.
Following digitalization of power grids, monitoring equipment is connected to the bushing taps to measure properties of the main insulation such as capacitance, C1, loss factor (tan δ) or partial discharge. Condition assessment in the field can reduce risk of failures, if performed properly. The most sensitive methods for condition assessment need to be performed offline. To avoid outages and follow trends real time at service conditions online monitoring equipment is used connecting to the tap of the bushing, increasing the total impedance to ground and hence increase the risk of failures in service, see example in right panel of Fig. 1. Furthermore, connection to the inner components of a capacitance graded bushing through the tap removing the cover from the tap, exposes the tap to a new physical environment controlled by the monitoring equipment. These factors should be carefully evaluated when considering the implementation of any monitoring equipment.
To mitigate some of the risks associated with external monitoring equipment, a device that protects the bushing from overvoltages in the tap region due to the added impedance from the external equipment has been developed.
Attend the 2025 INMR WORLD CONGRESS in Panama where bushings expert, Henrik Löfås of Hitachi Energy will focus on a special set-up built to replicate VFTs and the routine test that verifies the function of each delivered device. He will also present the functional design of the BTP together with full-scale tests and the routine test setup and review results from both full-scale VFT testing on a bushing with the protection device as well as results from the routine test will be shown.
