Power System reinforcements the hardanger connection

Transcription

1 Power System reinforcements the hardanger connection E. HILLBERG, A. HOLEN G. ANDERSSON L. HAARLA NTNU, (NO) ETH Zürich, (CH) Aalto University, (FIN 1 - The following minimum security criterion is applied in the Norwegian power system: a single fault may result in the disconnection of up to 200 MW load (500 MW in situations during maintenance), [3]. Summary In order to increase the transmission capacity and security of supply to the Bergen region in Norway, the Norwegian government granted the concession of a new 90 km 400 kv overhead line into the Bergen region in July This decision has been considered highly controversial since the new line is planned to go through the picturesque landscape of the national romantic region of Hardanger. Due to public resistance, the Norwegian government established four independent committees to evaluate the feasibility of a 70 km 400 kv submarine cable connection, as an alternative to the proposed overhead line. Not only the length of this cable was challenging but also the fact that it had to be laid at a depth of 800 meters in the Hardanger fjord. This paper describes the work performed by the committee nominated to assess the power system related aspects of installing the new transmission line as a submarine AC- or DCpower cable, and the consequences of increased cabling in the Norwegian grid. The full report of this committee is described in [5]. The paper includes an adequacy and security assessment of the power system in the Bergen region, with and without the new line. 1. Introduction It is a well-established fact that modern society is dependent on a safe and reliable supply of electrical energy. Several investigations show that the electricity power system is the most important infrastructure, on which all other systems depend. Most customers tolerate short interruptions, but long and widespread interruptions can have serious consequences on society and are related with great costs. This fact has clearly been established after the major interruptions affecting different systems around the world in recent decades, [1]. Many power systems worldwide utilise the N 1 criterion to operate the systems in a sound and reliable manner. This is also typically the case in the Norwegian main grid, but in some parts of the system, this criterion cannot always be fulfilled 1. One such area is on the west coast of Norway, the Bergen region, including the second largest city in Norway: Bergen. The annual consumption in the Bergen region is approximately 8.8 TWh, with a maximum peak load of 1.8 GW. Installed production has a total capacity of 1.0 GW and the region is normally connected to the main grid via two 300 kv transmission lines with an N 1 secure transmission capacity of around 0.9 GW. In 2010, a CHP (Combined Heat and Power Plant) with a total installed electric capacity of MW was put into operation. However, electricity as well as heat generation is for the plant only, and the current Norwegian governmental policy generally excludes thermal generation in the power market if the plant does not have CCS (Carbon Capture and Storage) technology implemented. There are a few exceptions where concession has already been given. Since the power generation in the region is primarily hydropower, with limited water storage capacity, the region is very sensitive to the level of precipitation. Several actions have been taken to decrease the vulnerability of the region: 4 No February 2012 ELECTRA electra 260.indd 4 13/01/12 16:09

2 Increased regulation of the power generation within the region, through market control by the Norwegian TSO (Transmission System Operator) Statnett, in order to limit the required power transfer to the region Implementation of System Integrity Protection Schemes (SIPS) in order to have an increased secure power transfer capacity to the region while keeping the effects of disturbances limited to controlled load shedding Radial operation of the system in times with limited generation capacity within the region, decreasing the risk of total blackout but increasing the risk of partial blackout. In addition to these actions, a new transmission line is planned, which would improve the interconnection between the region and the main grid. This paper is organised as follows: In section 2, the background to the work is described, with the methodology and methods described in section 3. A reliability assessment of today s situation and the reinforced system are found in sections 4 and 5, respectively. Conclusions and recommendations are included in section 6, with an epilogue of the work given in section background The Norwegian power system is a part of the Nordic synchronous system, which also includes the power systems in Sweden, Finland, and Eastern Denmark. The Norwegian system is strongly interconnected with Sweden in the south, and further north the system is interconnected with Sweden and Finland. There are also connections to Western Denmark and the Netherlands through HVDC links. The main transmission system in Norway is shown in Figure 1b. The Norwegian power system is facing a massive expansion, with plans to install or upgrade around 3500 km of power lines until The goal of this expansion plan is to increase the power transfer capacity and to ensure a reliable power supply in Norway, and to reinforce the grid in order to serve existing and planned international connections [3]. One focus area is the transmission system feeding Bergen and the surrounding region, in this paper referred to as the Bergen region, see Figure 1a. At present, this region is fed through two 300 kv lines, which constitute a limit on the power transfer to and from the region. During the winter, the region is characterised by high load and low precipitation, and restricted power import. On the other hand, during the summer, generation surplus is exported from the region. With plans of increased hydro and wind production, fast generation curtailment may be needed to avoid overloading in the case of line fault during heavy export situations. Fig. 1: The transmission system in a) the Bergen region (blue and red lines represent 300 kv and 400 kv lines, respectively, dotted and dashed lines are planned system extensions) [9] and b) Norway [3] 6 No February 2012 ELECTRA electra 260.indd 6 13/01/12 16:09

3 In order to increase the transfer capacity between the Bergen region and the main grid, thus improving the reliability of the power supply, the Norwegian TSO planned an overhead line between Sima and Samnanger, as indicated in Figure 1a. The new line would create a third connection to the region, enabling N 1 secure operation during periods with limited local generation, maintenance or upgrade of existing lines. The region with mountains and fjords where this line would be installed, the Hardanger region, is regarded a national heritage Due to massive resistance against the overhead line solution, the Norwegian government established four committees to assess the feasibility of a submarine cable connection. The tasks of the committees were to investigate the following: First committee: Technology, economy and other aspects related to the submarine cable connection Second committee: Power system related aspects of a cable connection Third committee: Consequences of a delay in the construction of the connection Fourth committee: Economic consequences on the society due to a cable connection The installation of such a cable is however not a trivial task; a depth of up to 800 meters, steep gradients, and a length of 70 km all form technological and economic challenges. The submarine cable would not completely replace the overhead line, since a 20 km overhead line would be needed to connect the submarine cable with the Samnanger station. Furthermore, large reactive compensation installations would be needed at both cable termination points, with considerable local impact on the landscape and community. The committees started their work in September 2010 and delivered their final reports to the government on 1 February 2011, see [4]-[7]. This paper describes the work performed by the committee nominated to assess the power system related aspects of installing the new transmission line as a submarine AC- or DCpower cable. The committee focused its work on how the power flow and the reliability of the power supply to the Bergen region would be affected by different solutions for the Sima- Samnanger connection. The results have been compared with an assessment of the present situation, in order to quantify the level of impact of the different solutions. A full description of the mandate of the four committees is found in [2]. 3. Methodology and reliability analysis models As shown in Figure 1a, the transmission system connecting the Bergen region to the rest of the Norwegian power system consists at present of two 300kV lines: North: Aurland Fardal Modalen South: Mauranger Samnanger These two lines, collectively referred to as the BKK corridor, are the main contributors to the reliability of the power system in the Bergen region. The line lengths used in this study, when quantifying the reliability level related to the north and south links, are 150 km and 100 km, respectively. In this study, simplified models, limited to the present and future interconnections in the BKK corridor, are used to quantify: the adequacy (power flow) of the transmission system, the probability (hours per year) that the Bergen region will face a situation where system is not N 1 secure (i.e. vulnerable to the loss of one of the lines feeding the region) the return period (years) of power interruption in the Bergen region due to faults on the transmission lines combined with insufficient local generation In order to assess the probability of power interruption in the Bergen region, statistical data describing the number of hours per year with reduced security has been used. The 8 No February 2012 ELECTRA electra 260.indd 8 13/01/12 16:09

4 Fig. 2: Actual number of hours during years where the N 1 security criterion was not satisfied in the Bergen region, [10] number of hours for the years with reduced operational security, i.e. where the N 1 criterion was not satisfied, observed by the TSO (Statnett) are presented in Figure 2. This data is used as a base to define the probability space (p) of having a reduced operational security: : 1300 h / 5 years p = : h / 1 year p = 0.15 The lower limit 0.03 excludes the year 2010, while the upper limit 0.15 assumes that year 2010 is a typical year. A reinforcement of the transmission system, including a third connection to the Bergen region (Sima Samnanger) is studied for the following solutions: Overhead line, dimensioned for 2000 MVA, approximate length: 90 km Submarine cable, AC, single set, dimensioned for 1000 MW, approximate length 70 km, in series with a 20 km overhead line (faults on this line have only minor influence and are neglected from the calculations) Submarine cable, AC, dual set, dimensioned for 2000 MW, approximate length 70 km, in series with a 20 km overhead line. The faults on this line have only minor influence and are neglected from the calculations in this paper) Submarine cable, HVDC, single bipole, dimensioned for 1000 MW, approximate length: 70 km cable and 20 km overhead line (faults on this line have only minor influence and are ne- glected from the calculations in this paper) Submarine cable, HVDC, dual bipole, dimensioned for 1500 MW, approximate length: 70 km cable and 20 km overhead line (faults on this line have only minor influence and are neglected from the calculations in this paper) The motivation for the reinforcement is to allow N 1 secure operation under different operating situations: In the winter season during periods with heavy import into the Bergen region and limited local generation available due to limited hydro storage During maintenance and planned upgrading of the existing 300 kv overhead lines into the Bergen region In the summer season with surplus local generation and heavy export from the Bergen region 4. reliability assessment of the present situation 4.1 System Operation Winter In Norway, when the day ahead power market is settled, the TSO (Statnett) will check if the N 1 operation can be maintained without exceeding transmission limitations. This may not be possible without buying so-called special regulating power from 2 - In 2010, the total number of hours with reduced operational security was actually around 1700, [9]. In this study however, 1300 h has been used. No February 2012 ELECTRA 9 electra 260.indd 9 13/01/12 16:09

5 the separate market for regulating power. The price for special regulating power can be high, and, according to [11], the accumulated cost to the TSO from mid December 2009 to 22 February 2010 was 36 M NOK (approximately 4.5 M ), in the Bergen region alone. Due to low levels in the hydro storage and the uncertainty of the precipitation and load in the remaining winter period, the local generation company (BKK) decided on 22 February to save water instead of selling regulating power. This meant that there was no more special regulating power available in the Bergen region. The effect of this was that the N 1 criterion could no longer be fulfilled, which is shown by the actual power flow on the BKK corridor displayed in Figure 3a. At this stage, Statnett considered the best solution to be a separation of the transmission system, dividing the Bergen region in two sub-regions, each fed by a single line (i.e. radial operation). A single line fault could thus have led to a power interruption of considerable size, up to 1750 MW of load with up to customers, [8]and [11]. The cumulative probability of power interruption can be described by the curve 3 in Figure 3b, where the assumed fault frequency is 0.8 faults per 100 km of line per year, [12], and the total line length where a fault may lead to interruption is 830 km 4, [11]. The estimated probability of interruption is %, depending on the duration of this situation as indicated in Figure 3b. Although the assumption may be discussed, it is obvious that the risk was substantial. 4.2 Power System Operational Security Assessment An assessment of the probability of future power interruptions in the 3 - Faults are assumed to be exponentially distributed, with cumulative probability of interruption calculated as: F(t) = 1 -e -λt Where λ is the total fault frequency of the lines that lead to interru ption. 4 - Radial operation increased the systems vulnerability to faults, and the region was considered to be affected by failures on a considerably larger number of lines than the afore mentioned BKK corridor. Figure 3: System operation winter : a) Actual power flow and transfer capacity limits (short time and continuous N 1 limit) on the BKK corridor [8] b) Calculated probability of a power interruption in the Bergen region related to faults on the feeding transmission lines [5] 10 No February 2012 ELECTRA electra 260.indd 10 13/01/12 16:09

6 Figure 4: Calculated probability of future power interruption in the Bergen region related to faults on the feeding transmission lines, without changes in the present state grid, production or load, for the two previously specified values of the probability space (p), [5] Bergen region, due to faults occurring during periods with reduced operational security, is performed using a simplified system model (limited to the present and future interconnections in the BKK corridor) and the probability space (p) defined in section 3. Since only permanent faults are presumed to lead to an interruption, the fault frequency used in these calculations is approximated to 0.1 faults per 100 km of line per year (according to fault statistics [12] approximately 10% of all faults are of a permanent nature). The results, presented in Figure 4, show an estimated probability of interruption between 4-20 % for the coming five-year period, depending on the probability space (p) of having a reduced operational security. For the coming 15-year period, the estimated probability of interruption is %. It is important to keep in mind that these calculations are limited to the probability of failures only on the transmission lines in the BKK corridor, during reduced operational security, for the scenario of an unchanged system from the present state. 4.3 System Integrity Protection Schemes System Integrity Protection Schemes (SIPS), such as load shedding, generation curtailment, and/or network separation schemes, are utilised in the Bergen region to limit the consequences of faults at times of reduced system security. The SIPS are triggered by events, such as the disconnection of critical lines, transformers or buses; or by system response such as line overloading, bus under-voltage or underfrequency. This implies that, in cases where the N 1 criterion is not fulfilled, the system operation depends on the efficiency and reliability of the automatic SIPS. Such an operation scenario is referred to by Statnett as N ½ secure, relating to the inadequacy regarding the N 1 criteria, although more secure than the N 0 operation. 5. reliability assessment of the reinforced situation 5.1 Power System Adequacy Assessment The N 1 adequacy of the reinforced network is assessed using a simplified model of the power system in the Bergen region 5. The loss of the Mauranger Samnanger line is identified as giving the worst-case post-fault loading of the two remaining connections. A high transfer scenario is studied, with an import of 1800 MW 5 - A comprehensive power flow study described in [13] verifies the results achieved by the simplified model No February 2012 ELECTRA 11 electra 260.indd 11 13/01/12 16:09

7 Figure 5: Results from simplified power flow calculations, [5], for grid reinforcements with: a) overhead line, and b) single set submarine AC cable The return period of power interruption is assessed here using statistical data for fault frequency and fault duration, see [12], [14], and [15]. The fault frequency (of permanent faults) for both an overhead line and a subto the Bergen region. In Figure 5, postfault power flow results are shown, with the Sima-Samnanger connection as both an overhead line and a submarine cable. Approximately 2/3 of the total transmitted power to the Bergen region would flow on the new connection, if it were to be constructed as an overhead line. In the case of a submarine cable, the power flow on the new connection would increase to over 5/6 of the total transferred power. The reason for this highly asymmetric power flow distribution is due to the low reactance of the cable compared to the parallel path. This means that the single submarine cable set, rated 1000 MW, would be overloaded in the studied scenario. A solution to create a better power flow distribution would be to install a phase shifting transformer in series with the submarine cable. In such a case, it would be possible to distribute the load flow between the remaining lines in order not to overload any of the connections. The results achieved using simplified calculations are supported by the more detailed study described in [13]. 5.2 Power System Operational Security Assessment Table I: Overview of thermal capacity for: N 0 (intact grid), N 1 (single fault), and N 2 (two independent faults); and return period for insufficient transfer capacity for studied grid reinforcement alternatives for the two previously specified values of the probability space (p) 12 No February 2012 ELECTRA electra 260.indd 12 13/01/12 16:09

8 Figure 6: Return period for power interruption for grid reinforcements, [5], with: a) overhead line, and b) dual set of submarine AC cables marine (AC or DC) cable is approximated to 0.1 faults per 100 km of line per year. The fault duration for a line is assumed to be between hours, while for a cable it is assumed to be between hours 6. The fault frequency and duration for a phase shifting transformer is assumed to be in the same range as a 70 km long submarine cable, i.e faults per year with duration of hours. Similar assumptions are made for the HVDC system (excl. cable), i.e faults per year with a duration of hours. In Table I, the assessed return period of power interruptions is shown, for the studied grid reinforcement alternatives, together with approximate thermal capacities of each alternative. Here, the repair times are assumed to 50 hours for lines and 1000 hours for cables transformer and HVDC sys- tem. The impact of the time span of reparation time is shown in Figure 6, where the case with one overhead line is shown together with the case of a dual set of AC submarine cables. 6. Conclusions and recommendations The analysis presented in this paper shows that the following reinforcement alternatives would give a very high reliability: one overhead line, a dual AC cable set, two bipole HVDC cables. Adequate reliability would also be achieved with the alternatives: a single AC cable set with a phase shifting transformer, a single bipole HVDC cable. 6 - Due to the limited statistical data, the repair time of sub-marine cables is represented by a wide time span. In this way, the significance of having a very long repair time is shown in the reliability assessment 7- Due to the uneven distribution of power flow between the cable and the remaining line. No February 2012 ELECTRA 13 electra 260.indd 13 13/01/12 16:09

9 A single AC cable set gives only a small benefit compared with today s system, mainly due to the uneven distribution of power between the cable and the lines. The overhead line and the dual AC cable set are both identified as robust solutions, based on established technology, and they satisfy the requirements for the future scenarios described in [13]. The solution with dual bipole HVDC does also satisfy these requirements, if a satisfactory control strategy is developed. This committee did not assess the cost of the different cable alternatives; however, evaluations described in [4] shows that the relevant cable alternatives have an expected cost in the magnitude of 4 to 6 times the proposed overhead line. Furthermore, socioeconomic consequences and possible justification of spending this money to avoid an overhead line is discussed in [7], however without stating a specific conclusion. Briefly summarized: the proposed overhead line provides sufficient transmission capacity, is robust and reliable, and is definitely the most cost effective alternative. 7. Epilogue After the committees submitted their final reports, the Norwegian government decided on 1 March 2011 to adhere to the previously given concession of the overhead line connection between Sima and Samnanger. This decision was mainly based on the security of supply situation during the winter seasons, combined with the fact that a cable alternative was expected to take at least five additional years to complete, further extending the current vulnerable supply situation. Additionally, the extra cost was of course not in favour of a cable alternative. Local impact from the quite large reactive compensation installations needed for HVAC, or alternatively the AC/DC converter stations needed for HVDC, showed that cable alternatives might also be controversial. Finally, opting for a cable alternative in Hardanger could have given precedence to submarine cables elsewhere in Norway where conflicts between network expansion and nature conservation are likely. The importance of media involvement in this concession procedure has been analysed in [16]. The report concludes that the Hardanger connection was the fourth most reported news in Norway during 2010, and the question is raised whether informal information channels are replacing formal procedures. Anyway, results from scientific based methods to quantify the fundamental power system aspects, such as adequacy and security of supply, must always be carefully considered in the decision process, also in controversial cases, but getting the message through might be a challenge. Bibliography [1] D. Cooke, Learning from the Blackouts - Transmission System Security in Competitive Electricity Markets, International Energy Agency (IEA), 2007, available at: [2] The mandate of the four committees related to the submarine cable solution (Norwegian: Mandatet for de fire utvalgene knyttet til sjøkabelløsning), webpage: www. regjeringen.no/nb/dep/oed/aktuelt/ nyheter/2010/mandatet-for-de-fireutvalgeneknyttet-t.html?id=613179, accessed: [3] Network development plan 2010 (Norwegian: Nettutviklingsplan Nasjonal plan for neste generasjon kraftnett), Statnett, 2010, available at: [4] Report by committee 1 of the submarine cable study (Norwegian: Rapport fra sjøkabelutredningen Utvalg 1: Teknologi, økonomi og andre forhold knyttet til en sjøkabelløsning), , available at: 14 No February 2012 ELECTRA electra 260.indd 14 13/01/12 16:09

10 [5] Report by committee 2 of the submarine cable study (Norwegian: Rapport fra sjøkabelutredningen Utvalg 2: Virkninger for kraftsystemet ved kabling), , available at: www. regjeringen.no [6] Report by committee 3 of the submarine cable study (Norwegian: Rapport fra sjøkabelutredningen Utvalg 3: Konsekvensene av at man trenger lenger tid på en ny overføringsforbindelse til Bergensområdet), , available at: [7] Report by committee 4 of the submarine cable study (Norwegian: Rapport fra sjøkabelutredningen Utvalg 4: Samfunnsøkonomiske virkninger av sjøkabelalternative), , available at: [8] Updated assessment of security of supply in the Bergen area winter (Norwegian: Oppdatert vurdering av forsyningssikkerheten inn mot Bergensområdet vinteren ), Statnett, , available at: [9] Assessment of the need of Sima- Samnanger (Norwegian: Vurdering av behov for Sima-Samnanger), Statnett, , available at: [10] A. Lont, Next-generation transmission network - the path to increased capacity and improved security of supply (Norwegian: Neste generasjon sentralnett veien til øktnettkapasistet og høyre forsyningssikkerhet), Statnett, , avai- lable at: [11] Documentation of operating conditions in the BKK area winter (Norwegian: Dokumentasjon av driftsforhold inn mot BKKområdet vinteren ), Statnett, , available at: www. statnett.no [12] Annual statistics 2009: Interruptions and faults in the kv network (Norwegian: Årsstatistikk 2009 Driftsforstyrrelser og feil i kv nettet), Statnett, available at: www. statnett.no [13] Draft Memo - comparison of solutions Sima-Samnanger (Norwegian: Utkast til notat - samanlikning av systemløsninger Sima-Samnanger), Statnett, [14] Grid disturbance and fault statistics, Nordel, 2008, available at: www. entsoe.eu [15] CIGRE Working Group B1.21, Third-Party Damage to Underground and Submarine Cables, 2009, ISBN: [16] A. Ruud, Case Hardanger - An analysis of the formal concession process and media coverage related to the applied overhead line Sima-Samnanger (Norwegian: Case Hardanger - En analyse av den formelle konsesjonsprosessen og mediedekningen knyttet til den omsøkte luftledningen Sima-Samnanger), SINTEF Energy Research, , available at: www. cedren.no No February 2012 ELECTRA 15 electra 260.indd 15 13/01/12 16:09