Figure 2.9: a) Moisture ingress into oil- lled joint,b)Low oil level in oil- lled joint
which move over the surface and after a while may cover the whole surface. The carbonized path created between the connector and the earth screen results in formation of the short circuit path and eventually failure.
Shrink/pre-moulded joints
Degradation processes in shrink and pre-moulded joints can be initiated if air- lled cavities exist in or at the surface of the material due to a poor shrinking process. Due to electric eld enhancement at the edges of these cavities, PD activity is started. PDs cause erosion, further degrading the material. Moreover, there are other aws such as knife cuts in the insulation created during the joint installation, presence of pollution in the insulation and gaps between cable-ends connected inside the joint, that can provide preliminary sites for degradation to start.
2.3 Root causes
Good understanding of the deterioration process helps in preventing premature failures in the cable system. Knowing the possible causes for defects, and further on the degradation, is also valuable information that can be used to optimize the component lifetime. Several factors are involved in actuating the degradation phenomena. Generally, these factors can be classi ed as defects induced in the insulation during manufacturing, handling, and in service periods.
Manufacturing defects
Manufacturing defects address the root causes that result in delivering defective cables and accessories due to design and engineering de ciencies. Generally, the type test is intended to prevent such problematic components, however, it can happen that the type test is not su cient to evaluate the quality of the joint. Poor treatment of the insulating material at the manufacturing stage may result
in cavities and other defects in the insulation. Mainly, such defects are identi ed during the production testing for examining the quality and factory acceptance test (FAT). Mostly, cables are delivered without defects with (in the past mainly) an exception of insulation material susceptible for the creation of water trees. Such water trees can appear in large scale along the insulation in a later stages. The water treeing may be initiated from small impurities hidden in the semiconducting screens or the main insulation.
Handling defects
Handling defects category includes all the defects imposed during the delivery and laying of cables and accessories. Incompetent handling during the transporta- tion of the cable and the accessories damages the insulation or the protection layers. etc., providing initial sites for degradation process to begin. Apart from transportation damages, the major fraction of damages to the insulation in this category are caused by lack of expertise of the personnel involved in assembling the cable system or lack of su cient assembling guidelines provided by the manu- facturer. Insu cient assembling knowledge can result in several problems such as misplacement of the conductors in a joint, too fast preparation of the joint which creates all types of defects, rough treatment of the joint which leaves knife cuts on the joint, and so on. These defects provide an initiatory factor for degradation of the insulation.
However, one should bear in mind, this may not only happen due to lack of expertise, but also, pressure and stress involved for fast utilization of the system, bad weather conditions and other similar factors have impact on the probability of creating defects during the handling and installation stage.
The site acceptance test (SAT) are developed to test the quality of the in- stallation, however, these tests are not always capable of revealing the potential created defects due to the fact that either they are applied with the wrong test parameters (for instance DC instead of AC) or they are simply not su ciently developed and/or designed to identify certain defect under certain circumstances.
In-service defects
This category covers the damages created in the insulation media after the system is put into operation. This group includes damages caused by either operational condition of the system, environmental condition of the system vicinity or those created by third parties.
Operational condition - Many defects can be created within the insulation because of its operational stresses (see [22]). Due to load cycling, the insu- lation encounters thermal and mechanical stresses which damage the insula- tion. Over time, as a result of chemical transformation within the insulation structure, insulation loses its dielectric strength and consequently may en- counter breakdown. For instance in case of paper insulated cable, such struc- tural transformation results in drying out of paper, decreasing its mechanical
2.3. ROOT CAUSES 23 strength, and leading to failure of the insulation. Temporary overvoltages can create insulation degradation on a small scale that in a later stage of life might give problems even under normal operation conditions.
Environmental condition - After laying the cable in soil, the environmental condition plays an important role in extending or shortening the lifetime of the insulation. Depending on the ambient condition of the soil the defects may be initiated, may develop further, or even can disappear. Variation in humidity content of the soil is one of the factors that have impact on the aging of the insulating material. A drop in soil water level increases the soil temperature which in combination with a raised temperature due to the load cycling in uences the operational conditions (see: [22]). It exposes the cable system to thermal and mechanical stresses which may induce defects in the insulation. Unstable soil makes the cables to sink especially in the peat, which results in mechanical stresses on the accessories. Such forces result in movement and misplacement of the conductors in the joint cast which induces mechanical degradation. Besides, the mechanical forces to the cable will have impact on the water tightness of the joint, which creates a path for moisture penetration into the insulation. Moreover, the lead sheath can corrode under certain circumstances which causes moisture ingress to the insulation as well. Flatness of the ground where the cable is buried in is also playing an important role in the development of defects in the insulation. If the cable is laid in hilly regions, then the PILC cable parts on the slopes might dry out (depending on the cable design). This is also the case when the cables are entering substations and go up to the terminations. In this case also the PILC cable and/or its terminations are exposed to possible paper dry out.
Third party forces - Apart from failures caused by operational and environ- mental conditions, third party's active or passive involvements can harm the insulation in dierent ways. Digging activities taking place in the vicinity of the cable may either directly damage the cable insulation or indirectly via vibrations created in the soil. Heavy tra c on the road also introduces vibrations in the soil. Such vibrations along the soil below which the cable is laid may cause displacement of the conductors as well as result in reduced water tightness of the joint. DC current from e.g. railways owing trough the earth may partly follow the cable outer metal sheath of the screen and corrode the metal sheath especially where the current exits the sheath. This will result in water ingress and consequently may lead to premature failure of the insulation.
Chapter 3
Partial discharge monitoring
Aging mechanisms leading to failure in cable networks can be classi ed based on their underlying physical interaction in thermal degradation, mechanical forces, electrical stresses and environmental conditions. All aging phenomena have in common that they result in a change in the properties of the insulation. Dif- ferent aging mechanisms can act simultaneously, as part of the overall insulation deterioration process. That means that there can be essentially dierent quan- tities that could be monitored to identify the degradation process and its stage of development. Yet, the best choice is the one which does reveal the dominant degrading phenomena, and at the same time is practical feasible to be employed on an economically realistic scale. In fact, this choice is of fundamental impor- tance anywhere in the eld of diagnostics. One of the common mechanisms related to failure of cable insulation media is partial discharge (PD) activity. Indeed, the fastest aging mechanisms in cables are associated with PDs [23]. Hence, diagnostic systems capable of monitoring this activity have attained signi cant popularity as a condition assessment tool, especially for high voltage (HV) and medium voltage (MV) cable systems [24, 25, 26, 27, 28, 29, 30, 31, 32, 33]. In the following sections the importance of PDs in cable insulation degradation is discussed. Next, the pros and cons of on-line versus o-line cable diagnostics will be highlighted. The focus will be on a speci c technique called SCG (Smart Cable Guard) developed for on-line condition monitoring of the MV power cables and employed during this research work as a diagnostic tool. This technique which has many advantages in- cluding the possibilities to identify defects in an eective way is further introduced in the last section.
3.1 PD activity and aging
PDs are small localized discharges in insulating media, which can occur due to enhanced electric eld in the insulation caused by discontinuities in the insulation. They can appear from defects within the insulation media as a result of poor de- sign or fabrication, bad installation, heavy service conditions (too high voltages,
too high currents), long term in-service time under normal operational conditions or third parties involvement during the operation time. PDs are not a full break- down, but only bridge a part of the insulation. They serve as an indicator for aging insulation, and they can also be part of the degradation process by eroding the insulation thus shortening its life time and even evolve into a complete breakdown. Defects resulting in PDs come in many forms, but have in common that once their size and the voltage over the defect, reach critical limits, the occurrence of rst free electron can trigger a PD. This is usually a fast event, initiating a relatively low amplitude high frequency transient current. This current extinguishes usually on a nano-second scale. In a cable a transient signal starts to propagate from the defect. The charge displacement by the PD results in an electric eld opposing the external eld in the defect. Whether it repeats itself depends on the applied voltage, the defect type and its stage of development. For AC voltages the external eld will change and a new PD can be initiated as soon as the voltage over the defect crosses the PD inception level again. Changes in material dimension during shrinkage or expansion of a void in the insulation on load cycling may ignite the PDs as well. For constant applied voltage (DC) charge leakage can result in re- establishing an electric eld causing repeated PDs. For instance, pulse numbers measured for cavities embedded in 15 kV class, 380mm2, XLPE cable is reported.
For a cavity of 0.2 mm, seven PD/day, a hundred PD/day in a 0.5 mm cavity and about a thousand PD/day in a 1 mm cavity can be monitored at nominal voltage [34]. Repetitive PD activities can result in a formation of conductive channels, a process called electrical treeing, in the dielectric material. Generally it is believed, that the growth of an electrical tree leads to breakdown as soon as the tree fully bridges the insulation medium. However, in reality, this may not be an immediate event. After the electrical tree bridges the insulation, thermal heating of the ex- isting tree branches in combination with further growth of the electrical tree can cause further deterioration of the material and lead to the ultimate breakdown. Moreover, repetitive discharge activity does not only electrically harm the insu- lation but also causes a lower mechanical strength as well as may cause chemical deterioration. Changes in the dielectric properties, e.g. in the form of increased conductivity can intensify the local electrical stress at the tree tips accelerating the degradation process.
The severity of PDs strongly depends on the type of insulating material. Espe- cially, the distinction between paper-oil and synthetic cable insulation should be noted.
In paper-insulated MV cables, the actual current load with its resulting heat and repetitive discharges cause permanent chemical changes within the af- fected paper layers and impregnating dielectric uid (Figure 3.1). Over time, partially conducting carbonized trees are formed. This places greater stress on the remaining insulation, leading to further growth of the damaged region, resistive heating along the tree, and further tracking. This eventually results in the complete dielectric failure of the cable and, typically, an explosion may occur. PDs generally dissipate energy in the form of heat which
3.1. PD ACTIVITY AND AGING 27
Figure 3.1: Chemical changes in paper layers and impregnating uid - MV PILC cable
may cause thermal degradation of the insulation, although the level is norm- ally low. Basically, paper-insulated cables can resist PD activity for a long time, even up to several years.
Generally, organic and extruded polymers are more sensitive to PD activity. Extruded insulation cannot resist even a low PD activity, let alone signi cant repetitive PDs. Preliminary PDs expand in a weak spot very fast, depending on the electric eld stress. The defect evolves from a small void or cavity into an electrical tree which grows along the insulation medium in a matter of seconds, hours or weeks and ultimately causes a full breakdown.
It should be noted that a majority of faults occurs in cable joints, which connect dierent cable sections. These joints come in many types (Chapter 2), each with a speci c sensitivity to PD activity.
Generally, partial discharges are classi ed in three main groups, namely inter- nal, surface and corona discharges. Internal discharges occur in a cavity within the insulation media which develops further to electrical trees as a result of cumulative PD activity. Figure 3.2 [23] shows the development of an internal defect to a tree which ends up in a full breakdown.
Surface discharge occurs along the dielectric interface. The formation of a conductive path along the surface of the insulation results in tracking which further converts to electrical treeing and eventually complete breakdown. The electric eld strength in the dielectrics where discharges occur at the surface is fairly low. Figure 3.3 shows the formation of tracking from a surface discharge.
Corona discharges mostly happen at sharp edges in a gaseous or liquid media due to the presence of an inhomogeneous eld. Such discharges are considered to be less harmful to the insulation as compared to the internal and surface discharges depending on the material.
Figure 3.2: Growth of internal defect from a cavity to electrical treeing (copied from [23])
Figure 3.3: Formation of tracking from surface discharge followed by inward electrical treeing.
In cable systems, the classi cation of the discharges leads to the following events: Corona in air or oil
Surface discharges between liquid and solid, liquid and gas, solid and gas, solid and solid (Figure 3.4 shows an example of surface discharges in the air lled area between 2 interfaces in cable)
Internal discharge in cavities and from treeing
Each discharge activity results in a speci c PD magnitude distribution and it leaves a speci c pattern with regard to voltage which will be further discussed in Chapter 4.