180 / 2018-08-26 03:06:25

Partial discharges and space charges under HVDC supply: how do they interact?

Partial discharges,HVDC,space charge

— A number of recent works, starting from scientific research, to industrial standards as the IEC 62895, focus their attention on measurements of partial discharges (PD) in HVDC conditions, assessing it as a valuable testing method for quality control and design of insulation systems. A theory explaining all the different phenomena occurring while stressing an insulation by HVDC field has not fully established, and several studies are currently being carried out in order to shed more and more light on this topic.
Those efforts are not only interesting from an academical point of view, but also from a practical one, since the currently suggested testing procedures for HVDC assets have not been entirely specified, leaving ample discretion to apparatus manufacturers or utilities.
PD testing has been adopted under AC as an essential criterion to assess reliability and commissioning of cables and accessories, and, indeed, has been employed as one of the few practicable on-site testing methods for cable systems to detect manufacturing defects, commissioning flaws and as a diagnostic tool able tom reveal the existence of the fastest accelerated degradation process in organic insulating materials, that is, partial discharges.
Regarding DC, PD inception conditions and activity are usually profoundly different from AC (but not necessarily less likely). Field distribution is depending on both insulation geometry and conductivity, which in turn is changing with temperature along insulation thickness (hence, e.g. the loading conditions of a cable).
A second aging accelerating factor that is typical in DC and absent in AC is space charge. In fact, in AC space charge is a non-issue because charges do not have enough time to build up and, hence, cause any field magnification (compared to the Laplacian, geometric field) inside insulation. Under DC space charge injected at the electrodes can be trapped and generate space charge clusters (often close to electrodes) which can cause important field distortion, further magnifying the maximum electric field value in insulation and promoting accelerated local degradation.
While the influence of geometry and temperature on the electric field is predictable to some extent (i.e. once the final design and thermal coefficient of conductivity of the dielectric are known), the same level of confidence is hard to be achieved for PD occurrence and for the influence of space charge accumulation. In addition, the two topics may be likely interacting, because PD amplitude and repetition rate can be influenced by both injected space charge and by the charge accumulated by previous PDs and trapped at the cavity/solid dielectric interface.
Based on the challenges introduced above, this work has the aim to make a step in the direction of better understanding what the effects of space charge accumulation on PD behavior can be, by means of space charge accumulation and PD measurements.
Multilayered specimens having a flat artificial internal cavity were manufactured from extruded samples of polypropylene-based materials. Those were then characterized using a Pulsed Electro Acoustic (PEA) system, under different electric fields and temperatures, in order to estimate the amount of space charge typically being accumulated under different stress values.
Under the same conditions, PD measurements were carried and put in relation to the previous results. This way the effect of space charge could be associated with the results of PD tests, with the aim of deriving a correlation between the influence of space charge on local electric field in the cavity and, hence, modify inception, intensity and repetition rate of PD.