THE NEW STOREBAELT HVDC PROJECT FOR INTERCONNECTING EASTERN AND WESTERN DENMARK J. NÄCKER R. RÖSSEL E.M. LEUTNER

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1 21, rue d Artois, F PARIS B4-104 CIGRE 2008 http : // THE NEW STOREBAELT HVDC PROJECT FOR INTERCONNECTING EASTERN AND WESTERN DENMARK J.P. KJÆRGAARD, C. RASMUSSEN, K.H. SØBRINK 1 SUMMARY J. NÄCKER R. RÖSSEL E.M. LEUTNER S. NYBERG Energinet.dk Siemens AG ABB AB Denmark Germany Sweden Denmark has two power transmission systems: one for Eastern Denmark and one for Western Denmark. Eastern Denmark is synchronous with the Nordel system, whereas Western Denmark is synchronous with the UCTE system. The construction of the Storebaelt (Great Belt) HVDC interconnection between Eastern and Western Denmark has now been approved after many years of discussion. The Danish power system has changed considerably over the last few years due to the high penetration of renewable energy sources and due to the decommissioning of several old power stations. These changes have made the HVDC interconnection technically and economically advantageous. A feasibility study has revealed that several aspects will contribute to the benefits of the new Storebaelt HVDC interconnection: Operational benefits from using the most economical power plants in Eastern and Western Denmark. Shared power reserves in Eastern and Western Denmark, thereby reducing Energinet.dk's total purchase of power reserves in the market. Counterbalance of power imbalances between production and demand in Eastern and Western Denmark across the Storebaelt HVDC interconnection. Improved market functionality achieved by reducing power producers' possibility of obtaining market dominance and thus increasing market prices. A contract for the Storebaelt HVDC project with a rating of 600 MW was signed in May 2007, and the interconnection will go into commercial operation in April KEYWORDS LCC HVDC, Light Triggered Thyristors, Multi HVDC Infeed, Cable Load Prediction System. 1 khs@energinet.dk 1

2 1 THE DANISH POWER SYSTEM The transmission system in Denmark is owned and operated by Energinet.dk, the Danish transmission system operator (TSO). The system is divided into two electrically separated systems, the Eastern system and the Western system. The Western system is synchronous with the European system, UCTE, via 400 kv and 220 kv AC transmission overhead lines. The Eastern system is synchronous with the Nordic system, Nordel, via 400 kv and 132 kv AC submarine cables. The Western system has two HVDC interconnections to Sweden, Konti-Skan pole 1 (380 MW) and pole 2 (360 MW), and three HVDC interconnections to Norway, Skagerrak pole 1 (250 MW), pole 2 (250 MW) and pole 3 (440 MW). Table 1-1: Overview of the installed production capacities Synchronous area Western System Eastern System UCTE Nordel Central power stations 3400 MW 3800 MW Local CHP plants 1700 MW 650 MW Wind power plants 2400 MW 750 MW Wind power penetration % 30-85% Combined heat and power penetration % 25-74% The Danish power system is characterised by a high penetration of distributed power sources, ie wind power and CHP (combined heat and power) plants. In the Western system, wind power penetration is 200%, which is calculated as maximum wind power production divided by minimum consumer loading, while in the Eastern system wind power penetration is 85%. As regards CHP, penetration is 136% in the Western system and 74% in the Eastern system. In order to provide sufficient short-circuit capacity and inertia in the power system, at least three central power stations must be in operation at any time in the Western and the Eastern systems, respectively. 2 THE NORDIC POWER MARKET The Danish power market is a part of the Nordic power market, and Denmark is a member of the Nordic power exchange (Nord Pool [1]), which is jointly owned by the Nordic TSOs. The Storebaelt (Great Belt) power interconnection is one of five electricity transmission projects that were recommended by Nordel [2] in 2004, and which are now in the process of being realised. The other projects that were recommended are a new HVDC link, Fenno-Skan2, between Finland and Sweden, a new HVDC interconnection, Skagerrak 4, between Denmark and Norway, a new line between central Norway and central Sweden, and, finally, a line between North and South Sweden. The new Storebaelt HVDC project was decided by Energinet.dk because it will yield economic benefits as a result of better utilisation of the power system, sharing of power reserves across the Storebaelt HVDC interconnection, synergies in a common regulating power market and better market performance due to more competition in the market. The Storebaelt HVDC interconnection is planned to be ready for commercial operation in April

3 Exchange of power on the Storebaelt HVDC interconnection The expected exchange of power on the Storebaelt interconnection has been estimated on the basis of energy balances in the Nordic system using historical hydro- and wind-power time series and on the assumption that the production at local and central power stations is optimised. The equivalent full-load hours for the Storebaelt HVDC interconnection are 4,362 h/year, and the loss hours are 3,551 h/year while operating hours amount to 7,237 h/year. Technology and rating Classic HVDC with line-commutated converters and VSC (voltage-sourced converters) were considered. In order to find the most economical solution, various ratings and technologies were considered as shown in Figure Million DKK./Year x 400 MW 1 x 600 MW 2 x 400 MW 2 x 600 MW 1 x 600 MW + 1 x 350 VSC Utility value Yearly expenses Figure 2-1: Utility value and expenses per year for the Storebaelt HVDC interconnection Because of the higher utilisation rate and the relatively higher losses in connection with VSC transmission, LCC HVDC turned out to be the most economical solution. 3 SYSTEM STUDIES The Storebaelt HVDC interconnection will be designed to operate together with the existing HVDC interconnections and the large shares of distributed generation, ie wind power and CHP in the Danish power system. In order to design the Storebaelt interconnection, dynamic system studies for the most critical and severe situations have to be investigated in order to access the overall system stability. For this purpose Energinet.dk has developed a network model of the transmission grid in Denmark which is connected to simplified equivalents of the transmission grids in the neighbouring countries of Germany, Norway and Sweden. It is not feasible to cover all possible situations so it has been decided to focus on four different loadflow scenarios designated A, B, C and D. These scenarios represent the most severe operating situations for assessment of the dynamic behaviour of the transmission grid with the new Storebaelt HVDC system. The scenarios are shown in Figure

4 Scenario A: Peak load Production, central power plants: High Production, wind turbines: Low Production, local CHP: Medium Scenario B: Low load Production, central power plants: Low Production, wind turbines: High Production, local CHP: High Scenario C: Low load Production, central power plants: Low Production, wind turbines: None Production, local CHP: None Scenario D: Low load, island operation Production, central power plants: Low Production, wind turbines: None Production, local CHP: None Flow direction on existing HVDC line: Flow direction on new HVDC line: Flow direction on existing HVAC line: Figure 3-1: The four scenarios A, B, C and D to be used for dynamic studies Type of studies The studies for the project can be classified in three groups. 1. Design studies 2. System stability studies 3. Control, protection and communication studies. 1. The design studies are required for the specification of the equipment and the main circuit parameter such as overvoltages, reactive power conditions, insulation coordination, AC filter performance and rating of AC breaker and interference conditions. 2. The system stability studies include a stability modulation frequency control (SMFC) study and a subsynchronous resonance (SSR) study and a multi HVDC infeed study. 4

5 3. The control, protection and communication studies will start in 2008, and the testing of the HVDC control system in the digital simulator will be finalised by mid HVDC CONVERTERS The HVDC project The tenders were invited for a 600 MW monopolar LCC HVDC cable system with metallic return cable, which can subsequently be expanded to a bipolar system, if required. The project is split up into different subprojects and subcontracts for HVDC converters, submarine cables, land cables, AC substations and civil works, respectively. The contract for the LCC HVDC converters and the DC cables was signed May 2007, and the contractual date for handover and commercial operation of the interconnection April HVDC ratings The ratings of the monopolar Storebaelt HVDC system is 600 MW, 400 kv DC and 1500 A DC. The HVDC converter stations located at Fraugde and Herslev are designed to transmit rated power in either direction. Additionally, the HVDC converters have a dynamic power transfer capacity of up to 810 MW in both power directions. The DC cable transmission system consists of 26 km land pole and returns cables and 32 km sea pole and return cables. A single-line diagram of the Storebaelt HVDC Interconnection is shown in Figure 4-1. HVDC Station Fraugde HVDC Station Herslev AC Bus Thyristor Valves Smoothing Reactor HV Cables (land + submarine) 16 km + 30 km + 10 km Smoothing Reactor AC Bus Converter Transformer 2 x Triple Tuned AC Filter 2 x Double Tuned AC Filter 2 x Triple Tuned AC Filter 2 x Double Tuned AC Filter Figure 4-1: Schematic single-line diagram Main circuit parameters The continuous rating is 600 MW without a redundant cooling system in service. With redundant cooling, a continuous overload of 105% of rated load is achievable. Additionally, the DC transmission permits a 2-hour overload of 1.05 p.u. rated power without redundant cooling and up to 1.1 p.u. with redundant cooling in operation. Furthermore, a 3-second overload of 1.35 p.u. is available for use up to maximum ambient temperature and for the maximum 2-hour overload as previous operating condition. 5

6 Reactive power and AC harmonic filters In order to balance the reactive power demand of the DC converter at 600 MW, a total reactive compensation of nearly 340 MVA Mvar of shunt capacitors is required per converter station. The compensation is subdivided into four filter sub-banks of rating, each to comply with the limitation on AC voltage change due to filter switching at 420 kv AC system voltage. The design of the triple tuned AC filters [7] has been chosen to satisfy harmonic performance requirements over the whole range of operation as well as a high degree of flexibility and filter redundancy. A TT 12/24/36 B DT3/12 AC Bus 420 kv, 50 Hz AC Filters Arrangement at Herslev or Fraugde Arr Fac1.1 L1 C1 R1 Arr Fac2.1 L1 C1 R1 A B A B R2 L2 C2 R2 L2 C2 R3 TT 12/24/36 DT 3/12 TT 12/24/36 DT 3/12 L3 C3 Figure 4-2: AC filters for Fraugde () and Herslev () Using a special control mode with increased firing/extinction angles, the reactive power consumption of the DC converter can be increased in order to limit the reactive power flow into the AC systems during light DC loads without additional shunt reactors. Converter transformer The converter transformer configuration comprises three single-phase three-winding transformers for each 12-pulse group. One spare converter transformer will be installed at Herslev station. Table 4-1: The converter transformer s main data Fraugde Herslev Rated power of converter transformer MVA Commutating reactance 14 % 14 % Rated voltage of line-side windings kv Rated voltage of valve-side winding of Y/y transformer kv 167.2/ / 3 Rated voltage of valve-side winding of Y/d transformer kv Required number of tap-changer steps -5/+16-5/+16 Size of one tap-changer step 1.25% 1.25% Insulation levels- line-side winding LIWL / SIWL - valve-side Y winding LIWL /SIWL - valve-side delta winding LIWL /SIWL kv kv kv 1300 / / / / / / 650 The leakage impedances were determined by taking several factors into consideration such as the permissible short-circuit current of the thyristor used and optimised ratio between rating and construction cost, etc. The selection of a load tap-changer range was adapted to the requirements of AC voltage variation range and valve capability of operating at high firing angles [6]. All the secondary bushings of the transformer will protrude directly into the valve hall. Both star and delta connections are made inside the valve hall, thereby eliminating the need for wall bushings and avoiding lightning surge stresses of the valves caused by direct strokes. 6

7 The thyristor valves The converters of the Storebaelt project are based on 4-inch direct light-triggered thyristor (LTT) technology. The thyristor valves are arranged in three towers per 12-pulse bridge, each representing one phase. Every tower consists of eight modular units, each of which includes 30 series-connected thyristor levels with auxiliary components (eg snubber circuit, voltage monitoring electronics and saturable valve reactors) (Figure 4-3). valve section n=1 valve section n=4 C g n=15 C sn R sn V L V Figure 4-3: View into an LTT thyristor modular unit The valve towers are suspended from the valve hall roof, and all joints between modules such as suspension insulators, bus work and piping are flexibly designed to allow maximum deflection. Cooling water and fibre optics are supplied from the top. All non-metallic materials used were selected in order to minimise the risk of severe fires. Capacitors are filled with insulating gases and oil is eliminated. Plastic materials for tubing and insulation have flame retardant self-extinguishing characteristics. Smoothing reactor The smoothing reactor to be installed outdoor is of the air-core dry-type design and therefore maintenance free. Considering the metallic return DC circuit configuration, the calculations indicated that a 300 mh smoothing reactor per station is an adequate size to avoid resonance at low order harmonics. This size of the smoothing reactor ensures fulfilment of further tasks as well, eg limiting the transient over-currents caused by DC side faults or commutation failures, avoiding discontinuous current operation at low DC currents, especially at operation with high firing angles at minimum load operation. The smoothing reactors are installed at the 400 kv DC busbar. The insulation level of the 400 kv busbars is LIWL/SIWL = 1175kV / 950 kv. 5 HVDC CONTROL AND PROTECTION The control and protection (C&P) system follows a hierarchical structure: SCADA level (operator control and monitoring level) 7

8 CONTROL (and protection) level FIELD level. The different levels are interconnected by powerful redundant serial communication links such as Fast Ethernet (IEEE 802.3u) and PROFIBUS DP (EN 50170). The use of standardised communication protocols, eg TCP/IP and UDP, allows easy adoption to specific requirements and flexible implementation of additional features such as a future bipole control and remote access for maintenance purposes. SCADA level For the operators, control and monitoring of the HVDC system the SCADA level in the local converter stations consist of the redundant HMI system, the remote-control interface (RCI) for communication to control centres and to the AC station controller, the transient fault recording system (TFR) and the inter-station communication system. The remote-control interface (RCI) uses the IEC protocol for communication with the control centres and the AC station. This Linux-based system runs on industrial PCs connected to the control systems via the redundant local area network (LAN). The RCI is designed as a redundant (hotstandby) system. Central GPS-based master clocks are used to provide a common time base for all interacting station equipment. Telecommunication equipment interconnects the LANs of the two converter stations and provides the interface to the control centers. Inter-station communication is routed via the Energient.dk WAN network. CONTROL level The control and protection level comprises the station control, pole control and DC protection and measuring systems. These systems, as well as the AC filter main protections, are based on programmable logic controllers The Pole Control is the heart of the HVDC control system where control functions such as power and current control as well as other related control functions are carried out. Generally the Station Control integrates the HVDC system into the existing power system and carries out administrative functions (e.g. managing the control authorities). The reactive power control as a part of the Station Control is implemented in an external AC station controller. The communication to the external AC Station controller is realized via RCI. The protective systems ensure that all potential faults are detected, selectively acted upon and announced. The protection systems are divided into main and backup systems. The protection is equipped with redundant systems where different principles cannot be used. FIELD level Redundant serial field bus systems according to standard IEC Profibus-DP are used for communication between the control systems and the decentralized digital I/O units. For each redundant control system (Pole Control, Station Control) separate redundant field buses (system 1, system2) are provided. I/O units of the type SU200 are used for data acquisition functions. The units have integrated SER functionality with time stamping (resolution 1 ms) and digital filtering for each binary hardware input. The central measuring system is connected to the different control and protection systems and providing the interface to a hybrid optical DC measuring system, see Figure 5-1. The AC and DC system quantities are transmitted to the various control and protection processors via a high-speed optical Time Division Multiplexing (TDM) bus. To achieve a very high availability and reliability, the 8

9 measuring system including the sensor heads is designed completely redundant and independent of the Pole Control and DC Protection systems. Figure 5-1: Hybrid optical DC measuring system 6 RELIABILITY The energy availability (EA) for both Storebaelt converter stations together is guaranteed to be more than 98.5% with a forced outage rate (FOR) of less than five outages per year. Fault-tolerant control systems, an intelligent redundancy and spare-parts philosophy combined with stringent quality standards ensure high component and system reliability. Additionally, intensive offsite tests are performed on the HVDC control and protection system (eg functional performance tests), to enhance the reliability, availability and quality to the highest possible level. The spare-parts philosophy requires only one spare part for each main component (transformer, filter components, etc.) stored at Herslev converter station for the use at both converter stations. The sensitive station elements are designed with redundant capacity or as dual systems (valve cooling system, auxiliary power system, control and protection systems, etc.). 7 HVDC CABELS The 400 kv HVDC pole cables are mass impregnated (MI), and the metallic return is a polymeric cable. Table 7-1: Summary of pole cable data Submarine cable Land cable Cable length 32 km 26 km Rated voltage 400 kv 400 kv Rated current 1500 A 1500 A LIWL 800 kv 800 kv SIWL 800 kv 800 kv Cross section 1700 mm mm Cable load prediction system The maximum power exchanged is limited to the maximum continuous rated current capacity of the HVCD cable. 9

10 The cable design and the continuous rated capacity of the cable are determined by burial conditions and maximum temperature at the burial depth. Cables installed in air have normally higher capacity than buried cables due to better cooling conditions. There is a thermal capacity in the cable and the ambient soil that allows for a specified short-time overload at specified times. During the cold season, the continuous transmission capacity is higher than the rated capacity, and the short-time overload capacity has increased further compared to the warm season. Temperature margins for increased continuous capacity and short-time overloading is possible if a cable load prediction system (CLPS) is installed in addition to the control system in the converter station. The CLPS allows the utilisation of available temperature margins to increase the capacity of the cable through continuous control of the cable real-time temperature. The CLPS software calculates in real time the existing temperature margins of the cable and the ambient soil are utilised without exceeding maximum allowed temperatures in the cables. Figure 7-1 shows a functional overview of the CLPS system and the control system of the converter. In this overview, SCADA is an interface system for the converter station that also stores measured values, facilitates presentation of tables, diagrams, etc., and the control is for power control of the converter bridge. All data calculated by the CLPS is sent to SCADA and treated as any measurement in the system. Figure 7-1: Overview of functional diagram Temperatures on cable surfaces and in the ground are measured with three redundant Pt100 elements. If the redundant Pt100 elements indicate different measured values, the measured values are invalid and an alarm is sent to the SCADA system. All calculated data is supervised with the local SCADA system in the substation or transferred to a dispatch centre from where the HVDC interconnection is operated. Besides the calculated real-time data, further alarms or orders are sent to the SCADA system: One temperature element erroneous Two temperature elements erroneous Five minutes left to ramp down Ramp down order. 10

11 If two elements are erroneous, the CLPS cannot calculate correct temperature profiles. In this case the predicted overload currents and maximum continuous current are changed to the contractually rated value of the cable. The contractual value is below the real-time maximum continuous value. If an overload is ordered, the real-time overload decreases successively towards maximum continuous current. Five minutes before the time for finished overload is reached, an alarm 5 minutes left is sent to SCADA. If the operator does not intervene, a ramp down order is sent to the SCADA system when the time for overload is finished. This order automatically initiates a ramp down to maximum continuous current. 8 CONCLUSION The new 600 MW Storebaelt HVDC interconnection is one of five electricity transmission projects recommended by Nordel, the body for cooperation between the Nordic TSOs. The HVDC interconnection will interconnect Eastern and Western Denmark, and it will improve the Nordic electricity power market in general and the Danish market in particular. The benefit will be gained by increased trade in the Nordic power market and by a more optimised utilisation of the Nordic power generation system. Additional gains will be achieved by improvement of the market performance. LCC HVDC and VSC HVDC technology for the Storebaelt project was evaluated. Due to the expected high utilisation of the Storebaelt HVDC interconnection and consequently the relatively high capitalisation of transmission losses for VSC HVDC, it was decided to realise the project based on classic LCC HVDC technology with a rating of 600 MW. The DC pole cable is provided with a cable load prediction system (CLPS) allowing for cable overloads at peaks larger that the rated cable current and larger continuous currents during colder seasons. The project started May 2007 and the Storebaelt HVDC interconnection is planned to go into operation April BIBLIOGRAPHY [1] [2] [3] [4] Cigré Guide WG B4-41: Systems with multiple DC infeed, Electra no. 233, August 2007 [5] J. Rittiger, D. Zhang, Yong Jing, Xiaocheng Wu, Zongming Du, Xiaoming Jin, Digital Simulation of AC/DC Hybrid Transmission System," in Proc IEE-PES/CSEE International Conference on Power Systems Conf. [6] K. Eckholz, P. Heinzig, H.V.D.C.-Transformers - A Technical Challenge," in Proc IEE- PES/CSEE International Conference on Power Systems Conf. [7] C. Bartzsch, H. Huang, K. Sadek, "Triple-Tuned Harmonic Filters - Design Principle and Operating Experience," in Proc IEE-PES/CSEE International Conference on Power Systems Conf. 11