CLNR Trial Analysis Real-Time Thermal Rating Underground Cables

Transcription

1 CLNR Trial Analysis Real-Time Thermal Rating Underground Cables DOCUMENT NUMBER CLNR-L131 AUTHORS Peter Davison, Newcastle University ISSUE DATE 22/12/2014 University of Durham, and EA Technology Ltd, 2014

2 Contents 1 Executive Summary Introduction Real-Time Thermal Rating of Underground Cables Introduction Sustained Rating Cyclic Rating Distribution Rating Cable Model Cable and Site Installation Details EHV Cable HV Cable LV Cable Nominal P17 Cable Ratings EHV and HV cables LV Site EHV and HV Site Installation LV Site Installation Sensitivity Analysis Ambient Parameters Influence of Ambient Air Temperature on Soil Temperatures Soil and Cable Temperatures Load Currents EHV HV LV Peak Currents Model Validation Sustained, Cyclic and Distribution Ratings Best and Worst Case Ambient Conditions EHV and HV Sites LV Site

3 9.2 EHV HV LV Comparison to existing P17 ratings Real Time Thermal Rating Results Real Time Thermal Rating Cumulative Distribution Functions Load Scaling Cable Temperature Analysis EHV Cable HV Cable LV Cable Final Load Scaling Results Conclusions References

4 1 Executive Summary This report details findings from the Underground Cable RTTR trials carried out as part of the CLNR project. Trials have been carried out at 3 separate voltage levels covering a wide variety of cable construction types. The CRATER and Dynamic CRATER cable rating tools provided by EA Technology Ltd. have been used to generate the ratings in this analysis. An analysis of the ambient parameters measured at each of the sites, in addition to a sensitivity analysis of the model has been carried out. Ratings in line with those from P17 [1] have been calculated to better represent the conditions at each of the monitoring sites for comparison. Increased visibilities of conditions at site are certain to give more accurate results than those from tabulated, pre-determined conditions. To that end, soil thermal resistivities, soil ambient temperatures and real-time loading values have been monitored. These will allow comparison, in particular against the typically used Industry standard Load Curve G, used in the calculation of the cable Cyclic and Distribution ratings. At the HV and EHV sites even in the best case scenarios of low load, low ambient temperature and low soil thermal resistivity a de-rating of the cable from the calculated P17 rating values appears to be necessary. At the LV sites, in the best case scenarios, an increase over the P17 values is possible. Both the EHV and LV cables are relatively lightly loaded, however the HV cable shows a peak utilisation of 76.69% of the Sustained P17 rating. Cumulative distribution functions of the observed RTTR values have been created, with percentiles chosen to be representative of the percentage time for which an RTTR can be used with and without some form of direct management. At the HV site, with the most conservatively chosen percentile of the RTTR distribution a peak load of 138.9% (352 A) of the original peak load can be accommodated. At the LV site with a similarly chosen percentile a peak load of % (642 A) of the original is possible. 3

5 2 Introduction This report details findings from the Underground Cable Real-Time Thermal Rating (RTTR) trials carried out as part of the CLNR project. Power system components are limited in their current carrying capacity by the conditions which surround them, in the case of Underground Cables, levels of ambient soil temperature and soil thermal resistivity all affect the ability to transfer power. Higher soil temperatures and soil thermal resistivities result in a reduced ability to transfer heat away from the cable s outer surface leading to reduced loading capability. Real time thermal ratings (RTTRs) rely on real-time ambient conditions and necessary modelling parameters to provide increased visibility with regards to asset capability. Within the scope of the CLNR project, three cable voltage levels have been monitored in 2 locations. 33kV and 6kV monitoring was carried out at the Rise Carr Primary substation site in Darlington, with 0.4kV monitoring at the Darlington Melrose LV Secondary substation. Equipment was installed at the LV substation in February 2013 and in November 2013 at the Rise Carr HV and EHV sites. Details of the installation are given here, however the lessons learned report which discusses the Underground Cable RTTR methods provides greater levels of detail. The data analysis period for this report covers from initial installation to the end of October Multiple cable and ambient parameters have been measured at each of the locations and will be discussed in this report. This report has used the CRATER and Dynamic CRATER modelling tools to generate the ratings of the various cable circuits under investigation. 4

6 3 Real-Time Thermal Rating of Underground Cables 3.1 Introduction As part of existing BAU network operation, 3 ratings exist for underground cables in the UK. These are the Sustained rating, the Cyclic rating and the Distribution rating. Each has a particular meaning and time of use but are all related [2] Sustained Rating This is essentially the steady-state rating of the conductor. The current rating is the maximum load that can be applied, such that the pre-determined maximum conductor temperature is not exceeded Cyclic Rating Since power system loading is typically non-constant and can often be expressed as a time-repeating cycle, appropriate increases can often be made to the Sustained rating, taking into account the variable nature of the network loading. A 24 hour profile of normalised hourly load values has been typically used (the industry standard Load Curve G) to generate cyclic ratings. This is the case in the presently used standard for the rating of underground power cables, P17. The Cyclic rating relies on prior calculation of the Sustained rating Distribution Rating The distribution rating follows on from the cyclic rating in its approach. For typical operation a cyclic scenario is present. An emergency then occurs at the peak of the cycle. The load increases whilst the shape of the cycling load is maintained. At the end of the emergency period the conductor temperature has reached its maximum. Typically (and also for this report) Distribution ratings are calculated for a utilisation of 50%, i.e. the pre-emergency peak is 50% of the emergency peak. 3.2 Cable Model Figure 1 Simplified Cable Model 5

7 In order to model the thermal response of the underground cable to changes in ambient parameters a model of the cable and its various components is necessary. For this report the CRATER software programme has been used to calculate the sustained, cyclic and distribution ratings of the cable, however, as a brief introduction, the various components of the cable will be discussed. An electrical analogy is often used to represent the various thermal exchanges both inside and outside of the conductor construction. Thermal components are modelled as resistances and capacitances and are formed into an RC ladder network, Figure 1 shows the simplified model of underground cable where capacitances have been ignored at the present. The components in circles represent the losses in various parts of the cable construction. W c represents the losses in the conductor, often expressed as the I 2 R losses. W d represents losses in the dielectric material (the insulation). W s and W a refer to losses in the sheath and armour respectively. The T components refer to the various thermal resistances, with T 1 referring to the thermal resistance between the conductor and the sheath, T 2 is the thermal resistance of the bedding between the sheath and the armour. T 3 is the thermal resistance of the outer serving of the cable and T 4 is the thermal resistance between the cable outer surface and the cable surroundings. 6

8 4 Cable and Site Installation Details Only basic installation details will be given here as the Lessons Learned report covering underground cables contains more information regarding the site topography and installation methods of the monitoring devices. 4.1 EHV Cable This cable construction details which have been used to generate cable ratings using the CRATER software are as follows: Three single-core cables 240mm 2 Aluminium stranded conductor XLPE insulation Installed in triplex formation, direct in the ground Copper screened (area: 35mm 2 ) Nominal circuit voltage of 33kV The modelled burial depth of the cable is 1000mm Polyethylene outer covering Max. Conductor temperature = 90 o C During the course of the trial, the EHV cable construction was altered to reflect the section which is most likely to provide the thermal bottleneck for the circuit. After this alteration the cable is assumed to have the following construction: Three-core cable 0.3 in 2 Aluminium stranded conductor Paper insulation Copper screened (area: 35mm 2 ) Nominal circuit voltage of 33kV The modelled burial depth of the cable is 1000mm Compounded jute and fibrous material outer covering Max. Conductor temperature = 65 o C 4.2 HV Cable Three-core cable 0.2 in 2 Copper stranded conductor Paper insulation Belted common lead sheath Installed direct in the ground Compounded jute and fibrous material outer covering Nominal circuit voltage of 11kV (CRATER does not contain a 6kV function) The modelled burial depth of the cable is 1000mm. Max. Conductor temperature = 65 o C 7

9 4.3 LV Cable Three-core PILS/STA cable 0.3 in 2 Copper shaped stranded conductor (modelled in CRATER as 185mm 2 ) Paper insulation Belted common lead sheath Installed direct in the ground Compounded jute and fibrous material outer covering Nominal circuit voltage of 0.4kV Max. Conductor temperature = 80 o C The modelled burial depth of the cable is 450mm 4.4 Nominal P17 Cable Ratings EHV and HV cables EHV and HV cable ratings come from P17 Parts 1-3 [1]. Only distribution type ratings are given for non-metric sized cables in P17. EHV (1) refers to the 240mm 2 single core conductors, whilst EHV (2) refers to the 0.3in 2 3 core cable. Unless otherwise stated, P17 comparable ratings calculated ratings are for the following conditions: Ambient Soil Temperature: 10 o C Soil Thermal Resistivity: 0.9 m.k/w Burial depth: 800mm Conditions at site are designed to represent the burial depth at site more accurately. These are not intended to be the finally calculated ratings for these circuits. This will be discussed in more detail in later sections which consider the best and worst case ratings, based on the actual monitored conditions at site. From this point onwards, the calculated values of P17 ratings at Site will be used as the nominal ratings. For the EHV Site, the limiting EHV (2) configuration will be used unless otherwise stated. P17 (A) Calculated P17 Rating in CRATER (A) Calculated P17 Rating at Site in CRATER (A) Sustained Cyclic Distribution Sustained Cyclic Distribution Sustained Cyclic Distribution EHV (1) EHV (2) HV LV Site P17 does not give figures for LV cables therefore a slightly different approach has been employed. Northern Powergrid uses a set of tabulated ratings for LV cables, rated at an ambient soil temperature of 15 o C. From usage of the CRATER model the ratings shown in the tables appear closest to the following conditions: 8

10 Burial Depth: 600mm (Nominal burial depth is given to be 500mm in the spreadsheet) Soil Ambient Temperature: 15 p C Soil Thermal Resistivity: 1 m.k/w LV Ratings (A) Sustained Cyclic Distribution LV Rating from NPg Tables NPg conditions calculated with CRATER (500mm depth) NPg conditions with modified burial depth calculated with CRATER (600mm depth) LV Rating Calculated with P17 conditions (10 o C Soil Ambient Temp, 500mm burial, Soil Thermal Resistivity = 0.9) LV Rating Calculated with P17 conditions (10 o C Soil Ambient Temp, 450mm burial, Soil Thermal Resistivity = 0.9) LV Rating Calculated with EATL conditions (450mm burial, Soil Thermal Resistivity = 1) For future analysis, the LV ratings shown for the P17 rating conditions of 10 o C soil ambient temperature, 0.9 Soil Thermal Resistivity and 450mm burial depth will be used as the nominal rating for this circuit. It is also important to note that the ratings quoted in the Northern Powergrid rating tables are not a fixed parameter, various de-rating factors taking into account differing soil ambient temperatures, burial depths and soil thermal resistivities are available, and give similar results to those calculated in the CRATER program for differing conditions. 4.5 EHV and HV Site Installation The following details show the various measurements which were made at the Rise Carr primary substation. Unless otherwise specified, the measurements are used for both the HV and EHV cables. Soil Temperature (Ambient) Located on the HV side of the Rise Carr primary substation, close to the boundary fence, in order to remove any influence from other cables on site. This is at a burial depth of 900mm. Soil Temperature (Close to cable) This is located 130mm above the uppermost of the triplex group of EHV cables. Located 180mm horizontally away from the HV cable. Sheath Temperature measured on the surface of both the EHV and HV cables. The EHV measurement is attached to the uppermost of the triplex cables. Soil Thermal conductivity This is located 620mm from the wall of the substation building on the EHV side of the compound. The tip of the probe is 530mm below the surface and measures both temperature and soil thermal conductivity. Soil temperature is also measured by the device at a depth of 415mm below the surface. 9

11 4.6 LV Site Installation Soil Temperature (Ambient) This is located at a burial depth of 1000mm. Soil Temperature (Close to cable) Located 60mm horizontally away from the cable at a depth of 450mm. Sheath Temperature measured on the surface of the LV cables. Soil Thermal conductivity This is located at a depth 400mm, Soil temperature is also measured by the device. 10

12 5 Sensitivity Analysis In order to model the thermal behaviour of the cable as a whole, the analysis must be broken down into two sections. The internal heat transfers of the cable are largely governed by the cable construction and therefore are not subject to this sensitivity analysis. The external conditions surrounding the cable affect its ability to transfer heat away from the surface, and a number of factors are involved. This section will consider three influential factors on rating, the soil thermal resistivity, soil temperature and the burial depth of the cable. For all three cables, the nominal rated conditions are an ambient soil temperature of 10 o C and a soil thermal resistivity of 0.9 m.k / W. For the EHV and HV cables the nominal burial condition is directly buried at 800mm, for the LV cable the depth is 500mm. For reference, Table 1 shows the minimum and maximum values of the two ambient variables (burial depth is fixed) under consideration at the EHV/HV and LV sites. Soil Thermal Resistivity (m.k/w) Soil Ambient Temperature ( o C) Maximum Minimum Maximum Minimum EHV and HV Site LV Site Table 1 Minimum and Maximum Ambient Values at the EHV/HV and LV Sites Figure 2 Effect of Ambient Soil Temprature on rating 11

13 Figure 3 - Effect of Soil Thermal Resistivity on rating Figure 4 - Effect of Cable Burial Depth on rating As shown in Figure 2 and Figure 3 ambient soil temperature and soil thermal resistivity have the greatest effect on rating. Reduction in soil thermal resistivity (for example by using sand as the cable backfill) is likely to be the easiest method for increasing the rating of a circuit. Lower burial depths give an increase in rating; however this sensitivity does not consider the additional correlation with increased ambient soil temperatures at lower burial depths. 12

14 6 Ambient Parameters 6.1 Influence of Ambient Air Temperature on Soil Temperatures This section studies the effects of ambient air temperature on corresponding soil temperatures at various depths. As expected, those soil measurements which are closer to the surface seem to display greater sensitivity to the measured air temperature. This is shown by the ambient soil temperature measurements (typically lower burial depth than other measurements: see previous section for depths) which are higher than those measured by the soil thermal conductivity probe in the Winter period, due to a reduced influence from surface behaviour. The converse is true in the Summer period, where the temperatures are now greater at the surface. Figure 5 Ambient Temperatures measured at the LV site Figure 6 - Ambient Temperatures measured at the EHV/HV site 13

15 6.2 Soil and Cable Temperatures Figure 7 Soil and Cable Temperatures at the EHV site Figure 8 Soil and Cable Temperatures at the LV Sites As is shown in Figure 7 andfigure 8 the temperature measured at the soil thermal conductivity probe records the highest of the soil temperatures at both sites. The soil temperature close to the cable and cable sheath temperatures show good alignment, with an offset present, due to the slight difference in their location (180mm for the EHV cable and 60mm for the LV cable). 14

16 7 Load Currents In order to calculate suitably accurate cycling ratings at the various UGC monitoring sites, it was first necessary to check the observed load currents for network abnormalities such as N-1 events. A number of such events were found, although their duration was found to be minimal in relation to the length of the whole data collection period. The following section shows the original collected data and the resultant profiles after data cleaning. Given the relatively short duration of network anomalies, the data was simply declared null for the relevant periods. 7.1 EHV Figure 9 EHV Original Monitored Currents Figure 10 EHV Filtered Currents 15

17 7.2 HV Figure 11 HV Original Monitored Currents Figure 12 HV Filtered Currents 16

18 7.3 LV Figure 13 LV Original Monitored Currents Figure 14 LV Filtered Currents The noticeable increase in load current after the 22 nd of July 2014 is due to reconfiguration of the network between the Darlington Melrose Link Box and the Harrowgate Hill secondary substation. 7.4 Peak Currents P17 at Site (A) Peak Load (A) Sustained Cyclic Distribution EHV (1) EHV (2) HV LV (post-reconfiguration) LV Table 2 Peak Currents at each of the rating sites 17

19 Percentage Present Utilisation at Peak Load Sustained Cyclic Distribution EHV (1) 21.39% 18.50% 16.49% EHV (2) 24.19% 21.21% 19.44% HV 79.69% 71.58% 65.99% LV 67.27% 60.09% 57.59% LV (pre-reconfiguration) 33.99% 30.36% 29.10% Table 3 Percentage utilisation at peak load As can be seen in Table 2, there is significant headroom on the EHV cable to allow for load increase with a peak utilisation of 24.19% of the sustained rating. The HV system is loaded beyond 50% capacity at peak, with a maximum utilisation of 79.69% of the sustained rating. The LV system is, as per the EHV system relatively lightly loaded, with a peak usage of 33.99% of the sustained rating. After network reconfiguration, where the load now contains that from the Harrowgate Hill substation the utilisation clearly increases, to a peak of 67.27% of the sustained rating. The cyclic and distribution utilisation factors are naturally lower as is to be expected. 18

20 8 Model Validation Figure 15 and Figure 16 show the measured and predicted sheath temperatures for the HV cable calculated in the CLNR field trials. The offset between the two parameters is more clearly visible in Figure 16. There appears to be good agreement between the two data series, with and offset of roughly 2 o C. Figure 15 Measured and Predicted Sheath Temperatures (HV Cable) Figure 16 Measured and Predicted Sheath Temperatures (HV Cable) 19

21 9 Sustained, Cyclic and Distribution Ratings At each of the underground cable monitoring sites sustained, cyclic and distribution ratings have been calculated based on the actual observed conditions. These are intended for comparison with the present P17 tabulated values. Sustained ratings have been calculated based on the worst and best case conditions shown at each site. Informed by the results of the sensitivity analysis discussed previously the worst case conditions are a high ambient soil temperature and high soil thermal resistivity. The sustained rating is essentially the steady state continuous rating of the cable which will result in the conductor maximum temperature being reached, potentially after a long period of time. Since the Cyclic and Distribution ratings are independent of the load profile magnitude, the Load Loss Factor (LLF) becomes the significant indicator. The LLF is defined as: LLF = i=0 I i 2 I MAX Where I i refers to the 24 hourly normalised load values. For reference the industry standard Load Curve G has a LLF of LLF s have been calculated for a weekly average dataset taken from all monitoring sites. The maximum and minimum weekly LLF values have been then used to calculate a range of possible Cyclic and Distribution ratings. Ratings have also been calculated for using the actual observed values of ambient soil temperature and soil thermal resistivity during the weeks of minimum and maximum LLF. Weekday and Weekend ratings have been calculated since there is often a significant difference between the loading patterns of weekdays and weekends. Figure 17 EHV Minimum and Maximum LLF Load Profiles 20

22 Figure 18 - LV Minimum and Maximum LLF Load Profiles 9.1 Best and Worst Case Ambient Conditions In order to determine the range of potential observed ratings the raw monitored data has been converted into a series of hourly data. The data is then split into a weekly dataset, subdivided into weekday and weekend datasets, to take into account the differing load profiles at these times. Average weekday and weekend soil ambient temperatures and thermal resistivities are then calculated for each week. The worst and best case conditions are then as follows, a combination of the highest weekly (weekday and weekend) values of soil temperature and soil thermal resistivity and the load profile which has the highest load loss factor. The best case conditions are the exact opposite of these conditions. The potential non-coincidence of high ambient soil temperature and soil thermal resistivity is not considered here, these ratings are simply intended to provide an indicator of the potential worst and best case ratings at each of the sites based on the monitored conditions, as opposed to pre-determined tabulated data EHV and HV Sites Weekday Soil Ambient Temp Soil Thermal Resistivity Soil Ambient Temp Soil Thermal Resistivity Worst Case Best Case Weekend Worst Case Best Case

23 Table 4 Best and Worst Case ambient conditions EHV and HV Site LV Site Weekday Soil Ambient Temp Soil Thermal Resistivity Soil Ambient Temp Soil Thermal Resistivity Worst Case Best Case Weekend Worst Case Best Case Table 5 - Best and Worst Case ambient conditions LV Site 9.2 EHV Weekday Sustained Cyclic Distribution Sustained Cyclic Distribution Worst Case Best Case Weekend Worst Case Best Case Table 6 EHV Best and Worst Case Ratings 9.3 HV Weekday Sustained Cyclic Distribution Sustained Cyclic Distribution Worst Case Best Case Weekend Worst Case Best Case Table 7 - HV Best and Worst Case Ratings 22

24 9.4 LV Weekday Sustained Cyclic Distribution Sustained Cyclic Distribution Worst Case Best Case Weekend Worst Case Best Case Table 8 - LV Best and Worst Case Ratings 9.5 Comparison to existing P17 ratings EHV Sustained Cyclic Distribution Sustained Cyclic Distribution Worst Case Best Case Weekend Worst Case Best Case HV Worst Case Best Case Weekend Worst Case Best Case LV Worst Case Best Case Weekend Worst Case Best Case Table 9 Sustained, Cyclic and Distribution ratings for best case and worst case scenarios Table 9 shows the best and worst case scenario Sustained, Cyclic and Distribution ratings at each of the sites. Values in red indicate where a reduction in the present P17 value would be necessary, if such a scenario was to occur. Perhaps as is to be expected, a reduction is necessary at all sites if the worst case scenario was to occur. Since this scenario occurs at a high soil thermal resistivity and a high soil ambient temperature, the coincidence of these conditions is potentially unlikely to occur. At the LV site, in the best case scenario, an increase can always be made to the P17 rating with a maximum increase of 16%. At the EHV and HV sites however, even for a best case scenario it appears that the cables at this site need to be de-rated. This is most likely due to the very high 23

25 values of soil thermal resistivity observed at this site, however since P17 contains de-rating factors for non-standard conditions this does not appear to be a particularly singular problem. 24

26 10 Real Time Thermal Rating Results Figure 19 All calculated RTTRs from the Underground Cable Trial Sites Figure 19 shows all RTTRs calculated at all of the monitoring sites. There are a number of periods in the data collection period which require explanation. From the initial point of data collection to the 21 st of August 2014 there was found to be an error in the calculation of the 30 minute RTTR calculations. This error was corrected between the 21 st of August and the 23 rd of September This is clearly seen by the step decrease in the HV and LV ratings. The EHV rating shows a significant step increase on the 10 th of June Further investigation is necessary to clarify the accuracy of these values. After the 23 rd of September significant changes were made to the cable construction and maximum rated temperatures. This was carried out to aid the ongoing powerflow management trials in the CLNR project. The following changes were made: The EHV cable construction was modified to represent the smallest cable on the circuit (likely to be the limiting factor). The maximum operating temperature of this smaller cable was also reduced at the same time to 20.5 o C. HV maximum operating temperature reduced to 26.5 o C. LV maximum operating temperature reduced to 23 o C. 25

27 Figure 20 Calculated RTTR values after corrections made to RTTR algorithm Figure 20 shows the step decrease and increase in the calculated RTTRs after corrections were made to the RTTR algorithm. The results for the EHV cable calculated in this period are likely to be incorrect. Figure 21 Calculated RTTR values after modifications to construction and maximum operating temperature details Figure 21 shows the calculated RTTRs after significant changes were made to the cable construction and maximum operating temperatures as part of the CLNR field trials. 26

28 10.1 Real Time Thermal Rating Cumulative Distribution Functions Since the calculation of the RTTRs was only deemed to be correct between the 21 st of August 2014 and the 23 rd of September 2014 this period has been taken to generate CDFs of the observed ratings at the sites. After modifications were made to the algorithm, there were noticeable step increases and decreases in the calculated ratings. Since the peak loads can occur at any point in the monitored data, it is perhaps not fundamentally correct to compare RTTRs for the August-September period against peak loads; however, as shown in Figure 20, the ratings remain relatively constant in the pre and post modification periods. Due to this, and for the purposes of attempting to quantify the potential contribution of RTTR for underground cables, a peak-load analysis has been carried out. Results for the EHV cable are for the final implementation of the smaller cable, with a reduced operating temperature. Since these results give significantly smaller results than those for the actual cable construction they are included here for only for completeness. Figure 22 Cumulative Distribution Function of RTTR values at the EHV site 27

29 Figure 23 Cumulative Distribution Function of RTTR values at the HV site Figure 24 Cumulative Distribution Function of RTTR values at the LV site P17 Rating Values at Site Percentage of time for which RTTR can be used Sustained Cyclic Distribution EHV (A) HV (A) LV (A) Table 10 Percentiles of observed RTTRs at the UGC sites 28

30 Potential Peak Load relative to original peak RTTR Percentiles EHV (A) N/A N/A N/A HV (A) % % % LV After Reconfiguration (A) % % % LV Pre Reconfiguration (A) % % % Table 11 Percentage increase in peak load based on RTTR Values The increase at the LV site (for the original substation data) is significant, with a potential capability to supply a peak load sized at % of the original peak, with the most conservative percentile of the RTTR distribution. As previously mentioned, the HV network is loaded to a relatively high percentage of its P17 capability at present. As predicted the potential increase at this site is the smallest, however still with the capability to support a peak load of 138.9% relative to the original peak. 29

31 11 Load Scaling Cable Temperature Analysis With the usage of Dynamic CRATER, the conductor temperature of the cable can be determined based on a set of circuit loading values. Dynamic CRATER takes a series of 12 weeks of half hourly data which can then be scaled or modified. For this analysis the measurement data from the trials has been converted into a series of half hourly measurements. The average values of soil thermal resistivity and soil ambient temperature have been calculated for the required periods. A series of 48 weeks of data split into 4 periods of 12 weeks has been analysed for the EHV and HV sites. A set of 72 weeks of data has been analysed at the LV site. Results are shown for each site individually showing the maximum conductor temperature observed based on the new loading conditions. These are then finally consolidated in one results table. Figure 25 Plot of the Dynamic CRATER output 30

32 11.1 EHV Cable For the EHV cable the maximum circuit rated temperature is 65 o C. New Peak Load (A) Maximum Conductor Temperature ( o C) Table 12 EHV Circuit 12 Week Maximum loads 31

33 11.2 HV Cable For the HV cable the maximum circuit rated temperature is 65 o C. New Peak Load (A) Maximum Conductor Temperature ( o C) Table 13 - HV Circuit 12 Week Maximum loads 32

34 11.3 LV Cable For the LV cable the maximum circuit rated temperature is 80 o C. New Peak Load (A) Maximum Conductor Temperature ( o C) Table 14 - LV Circuit 12 Week Maximum loads 33

35 11.4 Final Load Scaling Results New Peak Load (A) P17 rating at Site (A) EHV Maximum Minimum HV Maximum Minimum LV Maximum Minimum Table 15 Maximum and Minimum Possible Peak Loads after load scaling The minimum possible peak loads for the EHV and HV sites show slight reductions below the P17 sustained ratings, though this is to be expected as these P17 numbers represent a soil temperature of 10 o C and thermal resistivity of 0.9. Again, this is not to say that the ratings employed at the sites are incorrect, merely that caution is necessary when rating cable circuits and that with increased monitoring, more accurate results can be generated, avoiding the potential for overrating. In the converse scenario, the LV maximum loads are significantly higher than the P17 ratings due to the favourable soil thermal resistivities and ambient temperatures. 34

36 12 Conclusions Increases to the current levels of circuit loading have been shown to be possible, particularly when considering the calculated RTTRs. Due to the high level of soil thermal resistivity at the Rise Carr EHV and HV sites it is recommended that further analysis be carried out when considering the nominal ratings of these circuits. The ambient conditions at the LV site appear more in line with those which are tabulated in P17. Simply replacing the cable thermal backfill with materials of lower soil thermal resistivity (typically sand) appears to be a sensible first step when aiming to increase the circuit capacity of underground cables. As shown in the sensitivity analysis in this report, the value of ambient soil thermal resistivity greatly affects the cable rating. Whilst decreases are potentially required to the ratings (based on nominal P17 conditions) at the EHV and HV sites, the level of circuit utilisation at present indicates that this will not affect the maximum present loading capability. The results shown in Table 9 suggest that for the best case conditions, de-rating is potentially necessary, however these P17 rating conditions are representative of a much lower soil thermal resistivity. Whilst cables may need to be de-rated against these numbers, they simply represent the actual conditions at site, resulting in fewer opportunities for circuit overloading. 35

37 13 References [1] "Engineering Recommendation P17 - Current Rating Guide for Distribution Cables," [2] G. Le Poidevin, "STP Module 5 - S5196_3 - Dynamic Ratings of Underground Power Cables,"

38 For enquires about the project contact 4