B Operational experience of dynamic cable rating. E. JACOBSEN*, J. F. NIELSEN SEAS-NVE Holding A/S Denmark. S. T. SALWIN nkt cables a/s Denmark

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1 2012 Paris Session http : // B1-205 Operational experience of dynamic cable rating E. JACOBSEN*, J. F. NIELSEN SEAS-NVE Holding A/S Denmark S. T. SALWIN nkt cables a/s Denmark K.-H. COHNEN, A. MOHRS nkt cables GmbH Germany P. E. PEDERSEN Energinet.dk Denmark SUMMARY The future use of a load management system for dynamic rating of transmission cables was presented in paper B1-101, Cigré The load management system is based on distributed temperature sensing (DTS) of the cables using optical fibres. By combining the temperature measurements with the present and historical loading of the cable system it is possible to evaluate the actual loading capabilities as well as the short term overloading possibilities of the transmission lines. In the transmission system of Danish utility SEAS-NVE the load management system has now been installed on four 145kV lines including two export cable systems from offshore wind farms. This paper describes the experiences gained using a dynamic rating system as well as the system setup and the integration into SCADA system and daily operation. The 145kV transmission line, O120, is a 47km double circuit cable line with two land cable parts separated by a 28km submarine cable part. The system is monitored by a combination of short range multi-mode and long range single-mode based DTS systems. The line was commissioned in late The ampacity rating calculated by the load management system, based on 85 C conductor temperature, has always been slightly higher than the theoretical value from the cable design, indicating better thermal conductivity of the seabed than expected. The lowest ampacity rating refers to a section of the submarine cable. On the two export cable system of offshore wind farms unexpected hot spots have been detected. The export cable of Nysted Offshore Wind Farm, K120, installed in 2003, showed very poor thermal conditions on a section of the land cable part. The critical section has been cross bonded and ampacity rating is now significantly higher than estimated from the design. On the export cable of Rødsand 2 Offshore Wind Farm, N120, installed in 2009, a hot spot in the submarine cable part is detected. At this hot spot the ampacity calculated by the load management system is below the value from the design. * eja@seas-nve.dk 1

2 The load management system has been integrated into the SCADA system, delivering ampacity ratings for a number of time-ahead periods to the system operators as well as actual cable temperatures and corresponding positions. Further the load managements system gives alarms to the operators caused by various events like the load being higher than ampacity calculated, excessive temperatures, etc. KEYWORDS Transmission lines - High voltage cables - Distributed temperature sensing - Dynamic current rating - Grid management 2

3 INTRODUCTION In Cigré paper 2010 (B1-101) two projects in Denmark (N120 and N5/X5) were presented. On both systems the Valcap Load Management System is installed to monitor the thermal behaviour of the cable and its surroundings. At that time two additional systems were under construction, the K120 and the O120 system. This paper describes the setup of the systems and presents some results from the systems. O KV TRANSMISSION LINE The O120 transmission line consists of two cable systems in parallel with a total route of 47km, divided into two land cable parts with a submarine cable part in between. The system is monitored by a total of three temperature monitoring systems. The land parts are monitored by two multimode based 15km DTS systems while the submarine cable part is monitored by a 50km single mode based long range DTS System. Figure 1 shows the system setup of the O120 system. Figure 1:System setup of the O120 system The temperature profile of the O120 system shows a hot spot in the submarine cable part. Figure 2 shows the continuous ampacity (PCR, present continuous rating, based on a maximum allowed conductor temperature of 85 C) of one of the cables, the load, and the ampacity calculated by the cable manufacturer for the hot spot of the O120 system. Figure 2: PCR, current and max. load by cable design of the O120 system As the PCR calculated by the load management system is greater than the design ampacity, this hot spot is considered to be non-critical. 3

4 K KV EXPORT CABLE SYSTEM - NYSTED OFFSHORE WIND FARM The DTS system to monitor the K120 export cable was installed together with the cable system in 2003 and has now been included into the load management system. The DTS was originally installed for research purposes only. The K120 cable system is a 28km connection to the Nysted offshore wind farm and is divided into a 18km land cable part and a 10km submarine cable part. The system is monitored by a 30km single mode based DTS unit which measures the complete cable route. The route is divided into 85 sections with a total of 5 different laying configurations. Figure 3 shows the system setup of the K120 system. Figure 3: System setup of the K120 system The temperature profile in Figure 4 shows several hot spots. The highest temperature is between 2500m and 2600m. At this location the power cables is crossing a moor in pipes. Figure 4: Temperature profile K120 system From approximately 12000m to 18000m a significant drop in measured temperature is seen. The reason is that this section has cross bonded sheath earthing. The cross bonding was not originally installed on the cable system, but was added due to the initial DTS measurements. A severe hot spot was detected and analysed, the hot spot is still visible in the temperature trace at around 17200m. The conclusion was that the soil dried out at a lower temperature than expected combined with very local conditions related to significant drainage of the area. Due to the cross bonding, the area is no longer the limiting factor in the cable system. 4

5 The load management system now calculates the section with the lowest ampacity to be the landing part of the submarine cable. The calculated continuous rating (PCR) for this section is around 1000A. Figure 5 shows the load, the PCR and the ampacity calculated by the cable manufacturer for a time period of 4 days for the K120 system. Figure 5: PCR, current and max. load by cable design of the K120 project N KV EXPORT CABLE - RØDSAND 2 OFFSHORE WIND FARM The N120 cable system is a 38km cable connection from the offshore wind farm Rødsand 2. The cable system is monitored by two multimode based DTS systems each with 15km measurement range. Figure 6 shows the setup of the N120 system. One DTS is naturally placed in the onshore substation, RAD, while the other DTS is placed roughly 2/3 of the way of the total route (Intermediate in Figure 6). The latter measuring in two directions to monitor halfway back to the onshore substation (12km) and the rest of the cable line towards the offshore substation. The complete route is divided into 75 sections with six different thermal models like regular farm land, crossings, submarine, etc. Figure 6: System setup of the N120 system Based on the values of the N120 system in Jicable paper 2011 (C.9.2) the calculated continuous rating for the export cable was presented. At that time a value of approx. 910A for February 2011 was calculated by the AdapPro software for the detected hot spot in the export cable. The assumption was that during the summer months this value would change to a lower level. 5

6 Figure 7 shows the rating calculated by the load management system, the line load and the ampacity calculated by the cable manufacturer from mid of July 2011 until mid of October 2011 for the hot spot detected in the submarine cable of the N120 cable system. Figure 7: PCR, measured current and max. load by cable design of the N120 system The calculated PCR is around 815A which is approx. 100A lower than the value for February In Figure 7 the load on the cable systems was for several periods of time higher than the PCR predicted by the AdapPro software. Since those time periods are not longer than a few days the conductor temperature of the submarine cable never reaches a critical level. Figure 8 shows the measured temperature and the calculated conductor temperature for the same time period as in Figure 7. Figure 8: Cable temperature in the hot spot Figure 8 shows that the temperature measured by the DTS never reach a temperature more than 60 C whereas the conductor temperature was several times over 70 C. Based on the chosen alarm values for the N120 project (65 C for measured temperature and 70 C for calculated conductor temperature) the first alarm level was reached for the conductor temperature but not for the measured temperature. To find the reason for these high temperatures, additional evaluations were performed. For the period in time with high loads between and , the AdapPro software calculated based on the 6

7 measured current of 848A that after 7.6 days the conductor temperature will reach 90 C. The temperature after a 14 day calculation with 848A was 93.1 C. The PCR was 813A. In the past it was assumed that the reason for the hot spot is an area of peat soil. To verify this assumption a finite element simulation was performed to calculate which thermal resistance the surroundings must have for a calculated PCR of 813A to compare them with literature values. The simulation showed that for a thermal conductivity of 1.0W/mK and a current of 813A a conductor temperature of 85 C is achieved. The thermal conductivity for the seabed is normally assumed to be 1.43W/mK during the design phase of a submarine cable. The thermal conductivity for peat soil is between 0.55 and 0.8W/mK. DAILY OPERATION The load management system has been integrated into the transmission systems general SCADA system. The SCADA system delivers the real time currents of the systems and the load management system returns values of ampacity ratings for a number of time-ahead periods at suitable intervals. E.g. the possible line load for the next 3 hours is updated every 5 minutes, while the 100 hour ampacity is given every 6 hours. The forecast time and update intervals have been specified by the manager of the operational centre. The maximum temperature measured and the maximum calculated conductor temperature along with their corresponding positions is also reported to the SCADA system. The load management system is able to generate alarms or warnings on various events. These alarms are given to the SCADA system and presented to the operator like any other messages from the SCADA system. Events reported could be the actual current being higher than the 3 hour rating, high cable temperature, fibre breaks, internal load management system errors, etc. In mid-2011 the load management system was considered to be reliable and stable and the process of handing over responsibility of the system from the engineering department to the operational centre began. By the end of November the hand-over was completed, meaning that the operators now have to react on all messages from the load management system. In order to qualify the operators to use and understand the basics of the system, a training session for the entire operational centre staff was held. The training included introductions to the hardware, instructions and hands-on practice on how to use the software and especially when and how to react to the system alarms and warnings. The operators have full access to the complete system software, but in the daily operations the information from the load managements system is also presented on the SCADA system monitors of the general transmission system. The information presented can be seen in Figure 9 below: VALCAP system N5 MAV P120X5 MAV N120 RAD O120 STV SHEATH TEMP. > 60 degc (11) OK OK OK OK CORE TEMP. > 75 degc (12) OK OK OK OK CORE TEMP. > 85 degc (13) OK OK OK OK VALCAP ERROR (16) OK OK ERROR OK Rating (3 hours) Rating (24 hours) Rating (100 hours) Rating (2 weeks) Max system temp. (measured) Max system temp. position Max. conductor temp Max. conductor temp position Figure 9: Section of the operators screen 7

8 HARDWARE MAINTENANCE The load management system is hosted on standard server hardware specified by the company s regular IT department. The servers share hardware types and are monitored like all other servers within the company. This means that failing hardware like HDDs, CPUs, etc. is maintained and replaced together with the rest of the server park, giving the advantage of skilled IT personnel handling the hardware without it being noticed by the operational staff. The use of standard server hardware also gives redundancy as the servers work in parallel and many components can be hot-swapped. CONCLUSION From the engineering perspective this first period of operation of the load management system has revealed both unexpected hot spots and conditions better than foreseen in the design phase. Apart from the hot spot on the Rødsand 2 cable, the experience from the first years of operation reveals for all four projects a slightly higher ampacity than the design parameters would suggest. In general the land cable parts of the lines monitored show better performance than expected. The reason seems to be the design assumption on the thermal conductivity of the native soil being on the safe side. Also the soil conditions in the land cable routes are quite well known compared to submarine parts and unexpected conditions are more likely to be detected during installation. However, as was seen in the K120 Nysted Offshore Wind Farm project, very local phenomena does occur and might not be easy to spot even during installation. On the submarine routes the seabed parameters are more difficult and expensive to examine and the nature of the seabed material is not necessarily detected during the installation operation. On the other hand the seabed is usually more uniform than the soil on land. On the N120, Rødsand 2 Offshore Wind Farm, the hot spot is highly unexpected, as earlier analysis on the submarine export cable of neighbouring wind farm, Nysted, indicated somewhat better thermal conductivity than used for the design of Rødsand 2. An examination of the seabed around the area of this hot spot is at the time of writing under consideration. REFERENCES [1] E. Jacobsen, J. F. Nielsen, S.T. Salwin, et al., 2010, "Dynamic Rating of Transmission Cables (Cigré 2010, B1-101) [2] E. Jacobsen, J. F. Nielsen, S.T. Salwin, et al., 2011, "Dynamic Rating of Transmission Cables (JiCable 2011) 8