Electric fields in HVDC paper-insulated cables

Abstract

HVDC cables start playing a more and more important role in interconnecting national grids. This paper deals with the calculation of electric fields in HVDC cables. The calculation of fields in an HVDC cable is far more complex than the equivalent case in HV ac cables. This is due to the fact that the conductivity of the cable insulation is temperature and field dependent and due to the fact that the electric fields under dc voltage may be time-dependent. The field distribution in an HVDC cable may be of a capacitive, intermediate (and time-dependent) or resistive nature. The kind of field depends on the stage the cable finds itself in: for instance, whether the voltage has just been applied, whether a polarity reversal has occurred or whether the field distribution has become stable. For each stage, the method of calculating, together with the computed results on a real HVDC cable are discussed. Usually, the effect of heating of the insulation by the leakage current may be disregarded. However, in certain cases, i.e. the cable temperature and applied voltage are high enough, the field distribution is influenced by these insulation losses. They even may lead to an instability that causes breakdown of the cable. A cable in service may be subjected to impulses superimposed on the dc voltage. The most severe case is that of an impulse superimposed on a dc voltage of opposite polarity. The calculation of the field distribution in this situation also is carried out.

Grid capacity and efficiency enhancement by operating medium voltage AC cables as DC links with modular multilevel converters

“…The influence of partial discharges and their interaction with voids in insulation in the growth of trees is noted to be more of a high field phenomena expected to present a lesser degree of problem at medium voltage level [32]. Unlike ac operation however, the electric field distribution across cable insulation under dc conditions [35] can change with operating conditions such as temperature dependent field inversion and field enhancement due to polarity reversal. Also, the operational demands and therefore the transients occurring during dynamic operation and faults are different.…”

“…The electric field stress imposed due to operating dc voltage on the cable insulation exhibits temperature dependence [31,35], as the conductivity of the insulation changes with temperature. With increasing temperature and due to the fact that the dc electric field is unidirectional, space charges develop corresponding to the gradient in current density through the insulation that alter the field distribution.…”

“…The approximated expression for temperature dependent electric field stress E(x) on the cable insulation under dc operating conditions is given by (16) [31,35].…”

The coming of Age of HVDC extruded power cables

“…The electrical resistvity of the XLPE adjacent to the inner semicon may be much less than next to the outer semicon and so the electric field may be much higher near the outer part of the insulation than the inner -a so-called 'field inversion'. Jeroense and Morshuis considered this in some detail for HVDC paper-insulated cables [28]. Fig.…”

Analytical Calculation Method for Steady-state Current Capacity of HVDC Cables

“…Under the DC state, electric field distribution of insulation depends on the conductivity of cable, while conductivity affected by temperature and electric field strength [9]- [11] . For the calculation of electric field distribution in insulation layer, which involved in the steady-state current capacity calculation of HVDC cable, the analytical method proposed in [12], which requires temperature and electric field intensity at one point in the insulation layer as a reference. In [13], the iterative method is used to solve electric field, which proves that temperature difference of insulation layer is closely related to electric field distribution.…”