Abstract Electrical cables are the major arteries for electrical power. The power utilities are facing new challenges due to liberalization of electricity. It means reliability demand on distribution network is increasing day by day. The distribution networks are mainly cable networks of different voltage grade and dielectrics. Therefore, the proof design of cables is required after installations. During installation the wear and tear of the cables are must due to mechanical and environmental stresses.
Cables are buried in the ground, laid in conduits, hung down on the mines haft/trays/ducts them to survive the elements and man . Today ,it is possible to, on site, do cable diagnostics of all types of high voltage cable. This paper will mainly concentrate on PILC & XLPE cable of different voltage grade. There are three major categories of cable tests as noted below: (a) High Potential Withstand Test (HI-POT) (b) General Condition Assessment(GCA] (c) Partial discharge (PD)
In this paper, the emphasis has been given to the diagnostic method for VLF & TD, PD for PILC & XLPE cables. Key words-Water tree, Damped Alternating Current (DAC), Very Low Frequency (VLF) & Tan Delta (TD), ) Partial discharge (PD), Oscillating Wave Test Method (OWTS). 1. Introduction Liberalization of electricity markets brings about important changes in the working environment of power utilities. The power utilities are facing new challenges everyday pertaining to the reliability of power system.
It means reliability demands on distribution networks are increasing. To meet this demand in proper way, it is necessary to avoid shutdowns/breakdowns of the networks. To maintain the reliability demand of utilities, it is important to do right things at . right place at right time. As a result the network operators have to concentrate more on asset management to reduce the costs, to postpone the investment, to optimize technical management keeping at the same time the reliability and power quality at high level.
And to achieve the above cited goal i. e. avoid outage of supply, it is important to diagnose the network system regularly i. e. the diagnostic test to asses the condition of dielectrics of cables which are the major vital components of power system. Before switching over to field diagnostics, it is important to be acquainted with insulation condition assessment rules [5]. 2. General Aspect The actual ageing processes generated by the interaction of partial discharges are still not known well and understood at present time.
In particular, taking into account in distribution power cables important aspect etc. the detecting the defects can be used in combination with PD knowledge rules to support the condition best management. The important symptom of degradation process can be related to discharging local insulation imperfection or defects, which may occur in the particular cable part or cable accessories. Before performing diagnostic on the cable, it is important to know insulation defect for different type of power cable, shown in table -1 Table-1 Power Cable Insulation Defects Cable PILC |Accessories |Insulation | | |Low oil level |Damage outer sheath | | |Sharp edge on conductors |Tracking internal | | |Moisture penetration |damages | | |Air/ gas bubble |(as a result of | | | |bending) | |XLPE |Sharp edge on conductors |Damage outer sheath | | |Moisture penetration | | | |Air gas bubble | | | |Field grading movement | | | |Remaining semiconductor | | | |Interface problem | | | |Bad hardened resin | | This table clearly reveals the fact that approximately 40-45% of breakdowns in cable networks occur due to non-electrical external faults. This can be damages by digging activities. The 55-60% faults in cable network are caused due to internal electrical defects in cable joints and terminations of the cable. The general electrical defects, in high voltage cables on the basis of field experiences, are shown in table-2.
Going through the table-2, it is clear that any type of electrical faults in any type of Cables results in Partial Discharge which is cancer for the cable insulation systems. 3. Selection of modern diagnostics [2, 3&4] In tool box, there are three major tools i. e. HIPOT, GCA and PD. For many years, high voltage D. C. withstand test was only imminent diagnostic tools for testing of MV and HV cables networks, but now a days PD tool is applied for the same. HV DC Tool only deals with conductivity, and is blind for voids and cuts type defect. However, for some joints (resin filled) dc combined with PD gives good results. DC test for moisture penetration and PD for sharp edge. Table-2 The General Electrical Defects, in H. V Cables |Accessories |Interface problem> PD> Tracking. | |Bad hardening> Cracking> PD | | |Conductor problem> OH > PD | | |Local fault concentration> PD | |Excluded cable |Water tree> Electrical tree> PD | | |Voids> Delaminating> Electrical tree> PD | | |Local fault concentration> PD | |Paper/ oil |Oil leaks> Dry resin> OH> PD | |insulation |Water ingress> OH >PD | | |Local fault concentration> PD |
Moistures penetration in the resin insulation from out side to inside doesn’t result in PD due to conduction of insulating materials pertaining to moisture part, the rest (dry type) of the result is stressed with almost 90% of the applied dc voltage and breakdown. For the other type of joint and termination, other than resin type, PD testing at AC voltage stressing is the best tool option, and is applied for30 minutes test. As shown in table 2, PD is sensitive symptom of possible discharging weak spot (i. e. insulation defects, degradation in the high voltage insulation) [6] For water tree aged cable, PD detection is not effective tools. Other methods based on the dielectric response (i. e. eturn voltage, dissipation factor, dielectric spectroscopy, or Isothermal relaxation current) are better diagnostic tools [7] Partial discharge technique (analog) has been used to find manufacturing faults during acceptance test. It was stated that any discharge larger then a certain to measured below test voltage Vo leads to rejection of the object under test. In addition using phase resolved PD pattern the recognition defect is possible [8]. 4. Insulation and its ageing From physical point of view PD diagnostic test for a power cable should be performed under electrical stress most similar to service condition. In order to decrease the capacitive power demand for energizing the cable and hence to reduce the size ,weight and cost of the testing tools, the specific voltage shape have been introduced and employed since 1985.
The most common applied voltages are classified as shown in table-3. The following requirements can be defined for such diagnostic tool- (a)Non-destructive (b)Using standard and derived quantities (c) Providing distinction between different types of insulating problem (d) Providing PD site location and mapping. As a result, PD diagnosis has two ultimate purposes- a) to conduct on condition of the component measured b) to recommend- 1) about the change of cable or accessories 2) to perform more accurate on site visual inspection 3) to retest with certain time span 5. Factors to be considered prior to diagnostic test Table-3 Standard Method for Diagnostic Testing of Cable Network Voltage source |PD detection |PD information | |50 Hz online |VHF PD detection at power |PD activity in whole | |method |frequency voltage |circuit at V0 level | |50 Hz offline |Standard( IEC) PD detection at |PD occurrence & | |method |sinusoidal line frequency |location in the | | |voltage |selected cable section| | | |at different voltage | | | |method | |VLF method( |PD detection at 0. 1Hz |PD occurrence & | |0. 1Hz) | |location in the | | | |selected cable section| | | |at different voltage | | | |level | During construction/Installation, the different element can be used for cable networking. Each section may represent separate combination of elements like- a) Several different joints b) Two or more different cable terminations c) Different cable parts between particular joint/termination.
These elements affect insulation materials applied as well as different service conditions which may affect ageing in different way. In order to achieve a good result, it is necessary to have some information regarding the cable and network as mentioned below- a) Type of cable and accessories b) Accurate cable map c) Year of installation d) Current and past loading of cable e) Operating history and failing behavior f) Insulation condition g) Importance of networks. This information will be very useful for selecting the magnitude and wave form of voltage which is to be injected during diagnostic test. It is known that the XLPE cable has to be PD free.
Moreover, due to temporary over voltage occurring during network operation of the cable at Vo, service voltage has to stay PD free. As a result for the XLPE cable insulation, the PD inception and extinction voltage have to occur at higher level that the service voltage Vo. 6. Cables “fingerprints” The PD, in the cable section under test, generates several PD qualities which can be described into two groups – a) Base qualities: – Pd level in pC & nC, PDIV in kV, PDEV in kV, PD specimen. b) Derived qualities: – q-v curve, Phase resolved PD pattern, PD magnitude intensity. The above information which can be determined at different voltage level i. e. p to 2 Vo is collected and so called PD fingerprints of power cable section. The characteristics of these quantities may vary, measured on different cable section, in dependence on factors like age, service history & location of the elements used. 7. Interpretation basis The comparisons of the measured fingerprints which the database of the others measured cable section, gives the opportunity to criticality of a detected PD source and this way optimize the planning of the possible replacement of the cable part or accessories. In table 4. Schematic example of interpretation Rules for off line PD measurements on power cables have been summarized. Table-4 VLF Method for PD Diagnostics Power Cable PD Magnitude |High |Low | |(depending on | | | |correspondence) | | | |PD values |High |Low | |(depending on | | | |correspondence) | | | |PD density |High |Low | |(depending on | | | |correspondence) | | | |PD pattern |Dangerous type of |Less dangerous type | | |discharge |of discharge | |Network condition |Insulated neutral |Solid earth | 8. VLF Method [8] Actually this is ACHIPOT withstand test having very low frequency (0. 1 Hz to 0. 2 Hz) sinusoidal wave shape combined with dissipations factor measurements. ACHIPOT withstand test power frequency, is not applicable for on site test due to size and weight of apparatus results in transportation problem. At typical VLF of 0. 1 Hz, It takes 500 times less power to test the same cable compared to 50 Hz. Secondly, the magnitude of the tan-delta numbers increases as the frequency decreases, making measurement easier. Below equation shows that lower frequency results in higher tan delta numbers.
Tan Delta = True Power = 1 / ? cR Rad. Capacitive reactive power The block diagram given below shows the hardware assembly to perform the required diagnosis of high voltage cable. The block diagram me of VLF method is shown below-Fig. 1 Measuring unit consists of high voltage divider and fiber optically linked measurement unit. The high voltage divider measures the voltage and current input to the cable, sends this information to the controller which analyzes the voltage and current wave forms and calculates the tan delta numbers. AVLF unit is capable of testing from 1. 1 micro farad of cable load at 0. 1 Hz, up to 5. 5 micro farad at 0. 02 Hz.
For longer cables 0. 01 Hz VLF is also used. The very first test on cable yield valuable information about the insulation, also, most tan delta testing is performed on comparative basis. If cable’s insulation is perfect, the loss factor (Tan delta) will not change as the applied voltage is increased. The capacitance and loss will be similar with 1 Kv or 10 Kv applied to the cable. If the cable has water tree contamination, thus changing the capacitive/resistive values of the dielectrics, then the tan delta numbers will be higher at high voltage. Rather than a flat curve for loss numbers Vs Voltage, the curve will be non linear as shown in fig-2. [pic] From Fig. , we can see that the aged cable has extensive water tree damage. The loss angles increases with increasing voltage, indicating a high resistive current element to the insulation. These results can be compared to other cables. This is the most advanced tool for the diagnostics. [pic] Fig. 2 Voltage Vs Tan Delta Curve 9. OWTS Method [9, 10] The use of oscillating voltage for testing of installed cable has been under investigation throughout the 1990s. Modern technology innovations have finally enabled a commercial realization of the potential of this method by carefully considered integration of latest digital instrumentation techniques, solid state high voltage switching and Q HV components.
The cable under test is gently charged up to working Voltage, over a period of a couple of seconds, using DC source. At this moment a solid state switch with fast closer time creates a series resonant circuit from the test object and an air cored inductor. This circuit begins to oscillate at the frequency f =1/2?[pic]LC. The inductance of the air cone is selected such that the resonant frequency (within in the range of 50 to 1000 Hz) is similar to power frequency of service voltage. Medium voltage cable insulation usually has a relatively low dissipation factor and this combines with the low loss factor of air cone inductor to produce high Q (30 to 100) resonant circuit.
The result is an oscillating wave as the resonant frequency ‘f’ with decay time of 0. 3 to 1. 0 second. This produces a few tens of cycles to energize the test object and PD is initiated in similar fashion to 50 Hz inception condition. The block diagram of the OWTS system has been shown in fig. 3 [pic] This diagnostic kit has light weight (near about 100 kg) suitable for a site testing. Relevant aspect regarding the interpretation of the measurement data are discussed based on field measurement. [11]. Especially for XLPE cable, it is of importance to know in the relation to Vo of the cable voltage PDIV. PDEV. If the PDIV is larger than the service voltage of XLPE cable, PD can occur continuously during the service.
Due to decreasing voltage of oscillating PDIV can be determined due to the fact that for PILC insulation a certain PD level intensity is allowed. It is of importance to determine the criticality limits for PD activity. After discharges across such limit maintenance to be perform resulting in improving of insulation condition. OWTS method interpretation for PD of insulation condition. OWTS method interpretation for PD diagnostic has been shown in table-5. Table-5 OWTS Method Interpretation for PD Diagnostics |Inception& |< cable operating |< cable operating | |Extinction |voltage |voltage | |voltage | | | |PD Magnitude PD |
