Mini project TET4190

Power transmission to Ekofisk including offshore wind integration and transmission to UK

Group members: Thomas F. Johannessen, Bjørn H. Jørgensen, and Vegar Storvann

Contact person: Temesgen Haileselassi

Table of contents Table of contents ........................................................................................................................ 1 Abstract ...................................................................................................................................... 3 1. Introduction ........................................................................................................................ 3 2. Why wind power? .............................................................................................................. 3 3. Why connection Norway – Ekofisk – UK?........................................................................ 5 4. HVDC vs. HVAC............................................................................................................... 5 5. Current source converters and voltage source converters .................................................. 6

5.1. Current Source Converters (thyristor based).............................................................. 6 5.1.1. Basic principle of operation ............................................................................... 6 5.1.2. Converter station ................................................................................................ 7 5.1.3. Harmonics .......................................................................................................... 7

5.2. Self-commutated Voltage Source Converters (transistor based) ............................... 8 5.2.1. Introduction ........................................................................................................ 8 5.2.2. Offshore Transmission ....................................................................................... 9 5.2.3. Station Design and Layout ................................................................................. 9 5.2.4. Control and Operating Principles..................................................................... 10

5.3. Multiterminal HVDC ............................................................................................... 11 5.4. Filter requirements ................................................................................................... 11

5.4.1. Filters for CSC ................................................................................................. 11 5.4.2. Filters for VSC ................................................................................................. 12

5.5. Physical dimensions ................................................................................................. 13 6. Monopolar vs. Bipolar...................................................................................................... 14

6.1. Monopolar ................................................................................................................ 14 6.2. Bipolar...................................................................................................................... 15

7. Conclusion........................................................................................................................ 15 8. References ........................................................................................................................ 16 9. Sources ............................................................................................................................. 16

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Abstract This report deals with a scenario of electrifying the Ekofisk oil and gas field in the North Sea with a connection to the Norwegian grid, and at the same time providing grid reliability with a connection to the UK. Electrifying the Ekofisk field will reduce the CO2 and NOX emissions massively. It is assumed that an offshore 350 MW wind farm will be built nearby and connected to the same DC transmission to supply the platforms with renewable energy.

The connection will be done with a VSC-based multiterminal HVDC (high voltage direct current) transmission system. HVAC (high voltage alternating current) is not suitable for transmission across the distances involved in this scenario.

1. Introduction In this project report a scenario will be considered where a wind farm is placed in conjunction with the Ekofisk oil and gas field off the coast of Norway. A grid integration solution will be suggested.

In this scenario it is desirable to connect a HVDC line from the Norwegian land grid, via Ekofisk, to the British grid. The connection to the British grid is primarily for transmitting the surplus energy from the wind farm, while the connection to the Norwegian grid is mainly for stabilizing the energy supply at the platforms, as shown in figure 1.

Figure 1: Topology of the considered scenario

The distances on the figure are approximated distances from the Ekofisk installations to shore, connection points on land have not been considered. The size of the wind farm will in this consideration be about 350 MW. The number is based upon estimations of the power consumption for the Ekofisk field and the wind potential in the North Sea.

2. Why wind power? In the recent years there has been an increased focus on renewable energy and reducing the emission of greenhouse gases, especially CO2, in an attempt to reduce the greenhouse effect on the planet. Today, 78% of the global energy consumption is produced from fossil fuel, and only 19% from renewable energy [1]. It is important to increase the usage of renewable energy resources, both because at some point, the fossil fuel resources will be drained, and to reduce the greenhouse gas emissions. For example the European Union has decided that 20% of their energy consumption should be produced from renewable resources by 2020.

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Renewable energy is produced from naturally replenishing resources; this includes hydropower, wind power, solar energy, etc. The North Sea has a vast amount of wind energy potential, and this energy can be harvested by offshore wind farms. As a paradox, oil and gas platforms in the North Sea produce their own electricity from gas turbines, which produce large amounts of greenhouse gases.

The Ekofisk oil field in the North Sea has 29 platforms [2] and most of the electricity is produced locally by gas turbines, with the exception of the Valhall field, which is supplied with electricity via a 78 MW HVDC link from the Norwegian power grid. The Valhall HVDC link is said to reduce the CO2 emissions by 300 000 tons and the NOX emissions by 250 tons [3]. By electrifying the entire Ekofisk field, there’s a lot of potential for reducing greenhouse gas emissions.

Table 1: Ekofisk power consumption, collected from [5]

Load gas turbines [MW]  Annual CO2 emissions [1000 x tons] 

 

Total  Generators  Directly driven 

Total installation 

Electricity generation 

Ekofisk field  269  95  174  1 438  538 Ekofisk J  101  26  75  461  120 Ekofisk K  23  0  23  191  4 Eldfisk  54  8  45  262  55 Gyda  10  10  0  80  80 Ula  26  26  0  149  149 Valhall  51  23  28  267  121 Tor  4  0  4  28  9  As seen by table 1, the total potential for reduction of CO2 emissions related to electricity production is 538 000 tons. The maximum total power production, and then also consumption, is 269 MW.

By placing a large wind farm near the Ekofisk field, the electricity can be produced locally without greenhouse gas emissions and pollution. Wind power is a fluctuating power source, as the wind is not constantly blowing. Therefore a connection to the onshore grid is required to ensure a reliable supply of electricity.

Excess power from the wind farm can also be exported via the connection to the onshore grid. It might be more beneficial to export the excess power to a country that relies on electricity generated from fossil fuels or one that is a net importer of electric energy, both because it is more environmentally friendly and because the price of electric energy is higher in a system that’s a net importer of energy. Norway is usually a net exporter of electric energy, so one of the other nearby countries may be more beneficial to export to, like the UK or Denmark (and the rest of the continent).

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3. Why connection Norway – Ekofisk – UK? For the EU to reach its goal of 20% renewable resources it is necessary to have an efficient way of transferring energy between the involved countries. To that end several countries have agreed to build a high voltage power grid under the North Sea. The potential backbone of such a grid would be a high voltage direct current (HVDC) cable between Norway and the UK. The cable could be connected to nodes along the route, with the possibility of transporting energy from offshore generation and supplying low carbon electricity to oil and gas platforms in the North Sea.

The link offers several advantages by providing a connection between the Norwegian and British grids, allowing power flow in both directions and including the possibility for users and providers to connect along the route. It would give the possibility to tap into large scale wind energy, and the possibility for exchanging other renewable and low carbon energy generation like Norwegian hydro power and British low carbon coal power.

The link would also provide a better balanced supply and demand, reducing the intermittency that affects some renewable energy generation. An example is the option of storing energy produced at night from British wind farms in Norway’s hydropower facilities and releasing it the following day. This could have a big impact, with Britain’s goal to generate 15% of its electricity from renewable energies.

National Grid and Statnett have already carried out a study which suggest that a Norway – UK HVDC cable including the nodes along the route, could be economically and technically viable, as described in [9].

4. HVDC vs. HVAC When considering long distance transmission by cable connections, high voltage is an obvious choice, with lower power losses due to lower current. The dilemma is whether to use HVDC or HVAC.

The HVAC transmission does not require AC/DC converters at the ends of the transmission, but have problems caused by reactive currents. The maximum distance for HVAC cables is approximately 50 to 80 km caused by the cables capacitance [4]. The cables’ large capacitance causes high reactive current to flow in the cable, thus reducing the capacity for active power. The grid connected to the AC cable will also have potentially very high earth-fault currents in resonance grounded grids or grids with isolated neutral.

HVDC cable transmissions theoretically have no maximum length, and are well suited for long distance transmission of power. When using HVDC for transmission AC/DC converters with control systems are required at each end of the transmission. In addition, VAr compensation is needed for classical thyristor-based HVDC, since the valves of the converters require reactive power to operate.

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VSC-based (voltage source converter based) HVDC, however, does not require VAr compensation because it is capable of controlling both active and reactive power independently.

If one wants to inject or subtract power in the middle of an AC transmission, one would simply add a transformer and “problem solved”. In the case of an HVDC connection, this is not as easy and is discussed in more detail in chapter 5.3 about multiterminal HVDC.

Because of the distances involved in this scenario, HVAC cannot be used. HVDC is the only option.

5. Current source converters and voltage source converters

5.1. Current Source Converters (thyristor based)

Traditional HVDC converter stations are implemented using six-pulse thyristor converters (three-phase, full-wave converters, as in figure 2).

5.1.1. Basic principle of operation

A thyristor needs a current pulse on its gate to start conducting. It will conduct until the current between the anode and the cathode becomes zero. This makes it possible to control the output current or voltage of the converter. If the current pulse is delayed some amount of time after the thyristor’s natural point of conduction, the output voltage will become lower because the previous thyristor that was conducting was allowed to conduct longer than it would if there was no delay. This delay is called the firing angle, α, and is measured in degrees.

Figure 2: Six-pulse thyristor converter

When the firing angle increases, the current through the converter gets delayed by the same angle. This phase-shift between voltage and current makes the converter require reactive power to operate, and this is one of the drawbacks of this type of HVDC converters.

If the firing angle is increased past 90˚, the output voltage becomes negative and the converter is operating in inverter-mode. (This mode also requires reactive power).

Commutation (the transfer of current from one phase to another) in thyristor converters requires a strong AC voltage source in order to be successful. According to [6], thyristor converters require a three-phase symmetrical short-circuit capacity of at least twice the converter rating for the converter to operate.

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5.1.2. Converter station

Because of the high voltages involved (up to 800kV), one thyristor cannot withstand the entire voltage. This is solved by connecting many thyristors in series. One such series connection of thyristors is called a thyristor valve and is represented by one thyristor symbol in figure 2.

The thyristors’ switching causes harmonic currents on the ac side, and harmonic voltage components on the dc side. The harmonic ac currents need to be filtered, and this is done with large low-pass filters, as shown in figure 3. The filter banks are switched in and out as needed. When the dc power transfer increases, all currents on the AC side increase as well, including the harmonic currents, and more filtering is required. Filters are discussed more in detail in chapter 5.4.

It is normal to connect two six-pulse thyristor bridges in series to achieve a twelve-pulse converter. This requires two converter transformers (or one three-winding transformer) where one of the transformers is star-connected on both sides and the other is delta-connected on one side to get a 30˚ phase-shift between the phases for each converter. A twelve-pulse converter has fewer AC current and DC voltage harmonics. Figure 3 shows a typical HVDC CSC converter station layout.

Figure 3: HVDC CSC converter station layout, from ABB website

CSC-based HVDC converters are widely used today and are time-proven to be based upon reliable technology. According to manufacturers like ABB and Siemens, modern CSC-based HVDC converters have a capacity of up to 6000 MW at 800kV.

5.1.3. Harmonics

The harmonic frequencies of a six-pulse converter are 6n±1 times the line frequency for the ac current harmonics and 6n for the dc voltage harmonics. A twelve-pulse converter only has harmonic frequencies of 12n (DC) and 12n±1 (AC) times the line frequency because some of the harmonics will be 180˚ out of phase. This implies that a 12-pulse converter has lower filter

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requirements than a 6-pulse converter because it has only half the harmonics. To avoid harmonics in the current on the AC side, low-pass filters are used. The filters have capacitive components so they can also be used for reactive compensation.

5.2. Self-commutated Voltage Source Converters (transistor based)

5.2.1. Introduction

HVDC transmission using voltage source converters (VSCs) with pulse-width modulation (PWM), also known as HVDC Light (ABB) or HVDC PLUS (Siemens), has had a rapid progression to higher voltage and power ratings since its introduction in the late 1990s. Figure 4 shows a HVDC converter station with VSC.

Figure 4: HVDC VSC converter station layout, from [6]

These VSC-based systems use insulated-gate bipolar transistor (IGBT) valves, and are self-commutated. The self-commutation permits black start; this is achieved by having the converter synthesize a balanced set of three phase voltages, in other words having the converter acting like a virtual synchronous generator. Taking advantage of the dynamic support that the ac voltage at each converter terminal offers, one can improve the voltage stability and can increase the transfer capability.

Unlike conventional thyristor-based HVDC converters, the converters themselves have no reactive power demand. They can actually control their reactive power independently of the active power to regulate ac system voltage just like a synchronous generator. Figure 5 shows the active and reactive power operating range for a VSC station.

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Figure 5: Active and reactive power operating range for VSC converter, from [6]

5.2.2. Offshore Transmission

The advantages of self-commutation, dynamic voltage control, and black-start capability makes the HVDC transmission using VSC capable of serving isolated loads on offshore production platforms using submarine HVDC cables. This option eliminates the need for running expensive local generation, and increases the stability of the remote system. The VSC can operate at variable frequency making it able to efficiently drive the compressor and/or pumping loads found on offshore production platforms.

These advantages also make the system an attractive option for offshore wind farms. Offshore wind farms require a collector system, reactive power support, and outlet transmission. VSC-based HVDC transmission allows efficient use of long-distance submarine cables and provides reactive support to the wind generation.

5.2.3. Station Design and Layout

The transmission circuit is composed of a HVDC system with converters. The converters are connected to the ac system with the use of power transformers and ac phase reactors. Contradictory to conventional HVDC systems, harmonic filters are located between the phase reactors and power transformers. This allows the use of ordinary power transformers, since the transformers are exposed to no dc voltage or harmonic loading. To provide a stiff dc voltage source, capacitors are applied. Figure 6 shows the station arrangement for a converter station.

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Figure 6: HVDC VSC converter station arrangement, from [6]

The VSC converters use series-connected IGBTs. A complete valve consists of several IGBTs, an antiparallel diode, a gate, a voltage divider, and a water-cooled heat sink. To be able to switch voltages higher than the rated voltage of one IGBT, the transistors are connected in series. All IGBTs must turn on and off at the exact same moment to achieve an evenly distributed voltage across the valve. Higher currents are handled by connecting the IGBT components in parallel. The primary objective of the dc-side capacitor is to provide a stiff voltage source and a low-inductance path for the turn-off switching currents and to provide energy storage. The capacitor also reduces the harmonic ripple on the dc voltage. The ability to limit these voltage variations depends on the size of the dc-side capacitor. All equipment for VSC-based HVDC converter stations, except the transformer, high-side breaker, and valve coolers, is located indoors.

5.2.4. Control and Operating Principles

VSC converters used for power transmission permit continuous and independent control of active and reactive power. The active power can be controlled by changing the phase angle between the converters ac voltage and the filter bus voltage. The reactive power can be controlled by changing the fundamental component of the converter’s ac voltage with respect to the filter bus voltage. Being able to control these two aspects of the converter voltage makes operation in all four quadrants possible. This means that the converter can be operated in the middle of its reactive power range near unity power factor to maintain dynamic reactive power reserve for contingency voltage support similar to a static reactive power compensator. It also means that the real power transfer can be changed rapidly without altering the reactive power exchange with the ac network. Figure 7 shows the characteristic ac voltage waveforms before and after the ac filters along with the principle control of the transmission system.

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Figure 7: AC voltage waveforms before and after filtering for a VSC converter, from [6]

5.3. Multiterminal HVDC

Thyristor-based HVDC is the dominating configuration for point-to-point HVDC power transmission, but if one wants to connect several sources or loads to a HVDC transmission, VSC is the preferred configuration.

CSCs require complex solutions in order to be used in multiterminal HVDC, and are unable of supplying a passive load. Multiterminal CSC transmissions are in reality limited to three terminals [8]. In addition, CSCs have large reactive power consumption relative to the transmission power (50-60%) [7], which would require VAr compensation at both terminals.

When changing the direction of the active power flow for a CSC connection, the polarity of the voltage is changed. This requires the converters to be restarted, which takes approx. 1 sec. The direction of the current remains the same. For VSC transmissions the voltage polarity is not changed, only the current flow, which is done continuously. Because of this, VSCs are suitable for multiterminal HVDC.

5.4. Filter requirements

Both CSC and VSC HVDC converters generate harmonic currents and voltages. These harmonics can interfere with other equipment connected to the ac grid and can also cause interference with communication equipment nearby both the ac and the dc transmission lines.

5.4.1. Filters for CSC

12-pulse converters are assumed in this chapter.

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AC filtering

AC filters for CSCs are normally a combination of shunt-connected capacitor banks, reactors and resistors. The filters are tuned so that they represent a high impedance towards ground for the fundamental frequency, and a low impedance for the harmonic frequencies.

The harmonics with the largest currents in CSCs are the 11th and 13th order harmonics, so these are the most important to filter out, but the higher order harmonics are the ones that may interfere with, for example, communication equipment, so these need to be filtered as well.

DC filtering

CSC converters require filtering on the DC side as well. Here the order of the harmonics are given by the formula 12n, where n is an integer greater than 0. RLC filters are used here too, similar to the AC-side filters. A large inductance is also used to smooth the DC current.

5.4.2. Filters for VSC

AC filtering

High-power VSC converters have a switching frequency of up to ca. 2 kHz, and will because of this only generate high-order harmonics, as can be seen by figure 8. High frequency harmonics are easier to filter, and thus smaller filters are required than for CSC converters.

Figure 8: Harmonic spectrum for VSC converters, from ABB

A large inductor (reactor) is used to smooth the phase currents between the filters and the IGBT valves.

DC filtering

On the DC side, only a capacitor bank is required to smooth the DC voltage.

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5.5. Physical dimensions When placing the stations offshore, size and especially weight is of particular matter. The CSC’s valve bank takes more volume than the compact valve arrangement used in VSCs.

The CSC configuration requires filters both for dealing with harmonics and the reactive power consumption, while the VSC configuration only needs a small filter. This is because the line commutated thyristors require a lot of reactive power for switching and causes a lot of harmonics, whilst the CSC creates its own waveform with fewer harmonics, immensely reducing the demand for filtering.

The VSC has, as mentioned before, a greater demand for cooling as it dissipates more heat with its high switching frequency. Still, due to the greater demand for filters and reactive compensation for the CSC, the VSC is smaller and weighs less than the CSC.

An example of footprints: CSC station: 200m x 120m x 22m (600MW) VSC station: 120m x 50m x 11m (550MW) [8]

The stations in this example are placed onshore. The CSC station has an outdoor AC switchyard and high valve towers (22m). The VSC station has both the AC and DC yard indoors.

Figure 9: Comparison of CSC and VSC, from [6]

Figure 10 illustrates the size of a 2x40MW VSC converter on the Troll A platform.

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Figure 10: VSC HVDC converter on the Troll A platform

6. Monopolar vs. Bipolar

6.1. Monopolar There are two main configurations for HVDC transmission systems, monopolar and bipolar. In the monopolar configuration, one of the converter station’s terminals is grounded and the other has either a positive or a negative voltage. Only one transmission line or cable is required for this, as seen in figure 11.

Figure 11: Monopolar configuration, from [10]

The return through earth may cause some problems. In seawater it may produce chlorine or otherwise disturb the chemistry of the sea water due to electrolysis of the seawater. In the ground, corrosion of nearby underground metal constructions like pipelines may occur. A metallic return conductor can be used to prevent this.

As only one conductor is required, this configuration is cheaper and it has lower losses due to the high conductivity of the earth.

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6.2. Bipolar In the bipolar configuration, the two terminals of the HVDC converter have different polarity, one is positive and the other is negative. This requires two transmission lines or cables. This configuration is shown in figure 12.

Figure 12: Bipolar configuration, from [10]

The bipolar configuration can transfer twice the power of the monopolar configuration due to the two poles. In case of a broken cable or other faults on one of the poles, it can operate in monopolar mode (at half the rated power and voltage).

7. Conclusion Converter stations for CSCs require both a strong AC grid and reactive compensation for dealing with the valves’ reactive power consumption (50-60% of transmission capacity). The AC grid’s 3-phase short circuit capacity must be at least twice the DC transmission capacity. Therefore CSCs cannot be used as the only supply to a passive load, making it a bad choice for this scenario. Additionally, the converter stations for VSCs will both have a smaller footprint and require fewer components, which is critical on an offshore platform.

With VSCs, multiterminal operation is also an option, reducing the number of converter stations required for the scenario considered. Multiterminal operation also allows for future expansion of the DC grid. We therefore propose a solution using multiterminal VSC converters.

Figure 13: HVDC grid

The transmission capacity of all the HVDC converters and cables should be 350 MW (the size of the windfarm) to be able to supply the platforms and distribute the surplus energy from the

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windfarm if a fault should occur at any end of the transmission. Converter 1 and 3 can be built with a larger capacity to also be able of transferring energy between the two grids.

Because of the high reliability requirement for offshore installations, a bipolar configuration should be used. In case of a fault on one of the poles, the other can still operate and supply critical equipment at the offshore site at half the rated power and voltage of the converters.

8. References [1] Renewable Energy Policy Network for the 21st Century (REN21), Renewables 2010 Global Status Report, available at: http://www.ren21.net/globalstatusreport/REN21_GSR_2010_full.pdf, 2010 [2] ConocoPhillips Norway, The Ekofisk Area, http://www.conocophillips.no/read.aspx?db=internet/main.nsf&uid=BC191B5490ADC902C1256CDA004D0349, accessed 05.10.2010 [3] ABB AB Grid Systems – HVDC, Powering Valhall platform with HVDC Light, Available at: http://library.abb.com/global/scot/scot221.nsf/veritydisplay/70bc45517cce24cdc125755300532ae0/$File/POW-0049%20Valhall%20rev5%20LR.pdf, accessed 05.10.2010 [4] Siemens, HVDC Plus Power Transmission OWEN workshop, available at: http://www.owen.eru.rl.ac.uk/workshop_4/pdfs/owen_siemens03.pdf, 11.2000 [5] Thomas Palm, Et krafttak fra land, available at: http://www.zero.no/publikasjoner/et-krafttak-fra-land, 2007 [6] M.P. Bahrman, B.K. Johnson, The ABCs of HVDC Transmission Technologies, IEEE Power & Energy Magazine March/April 2007 Vol. 5 No. 2 [7] J. Arrillaga, Y. H. Liu, N.R. Watson, Flexible Power Transmission: The HVDC Options, August 2007 [8] The Crown Estate/Econnect Consulting, East Coast Transmission Network Technical Feasibility Study, available at: http://www.thecrownestate.co.uk/east_coast_transmission_network_technical_feasibility_study.pdf, accessed: 17.10.2010 [9] National Grid to investigate potential electricity link with Norway, New energy focus, 07.10.2009, available at http://www.newenergyfocus.com/do/ecco/view_item?listid=1&listcatid=32&listitemid=3070 [10] Wikipedia, High-voltage direct current, available at: http://en.wikipedia.org/wiki/High-voltage_direct_current, accessed 25.10.2010

9. Sources N. Mohan, T.M. Undeland, W.P. Robbins, Power Electronics: Converters, Applications, and Design, John Wiley & Sons; 3rd Edition (8 Nov 2002)

L.P. Lazaridis, Economic Comparison of HVAC and HVDC Solutions for Large Offshore Wind Farms under Special Consideration of Reliability, available at: http://www.ee.kth.se/php/modules/publications/reports/2005/X-ETS-EES-0505.pdf

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J. Blau, IEEE Spectrum: Europe Plans a North Sea Grid, available at: http://spectrum.ieee.org/energy/the-smarter-grid/europe-plans-a-north-sea-grid, accessed: 18.10.2010