DC Nodes for an Offshore SuperGrid
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Caitríona E. Sheridan DC Nodes for an Offshore SuperGrid Motivation An offshore grid in the North Sea has the potential to increase the penetration of renewable energy in Europe and to improve international energy transfer between European countries. It has been long acknowledged that this could be a precursor to a pan European SuperGrid [1]. It has been well established that HVDC technology will be used to transport this energy, given its suitability to long distance submarine cable transmission compared with HVAC. Furthermore voltage source converters (VSC) is more suitable for realising multi-terminal DC (MTDC) grids as power flow reversal is achieved by reversing current polarity as opposed to voltage polarity with classic HVDC technology. However there are several challenges with creating a MTDC grid. This research focuses on the technical aspects of how a MTDC grid could be developed using nodes to route power flow. It will investigate several technologies for use in such a grid and determine potential optimum ar- rangements for these nodes. The case study system will be the Dogger Bank Round 3 project. Fig 2: Potential MTDC grid and node arrangements for the North Sea A) AC arrangement B) DC arrangement This work is funded by National Grid UK and Ove Arup & Partners Contact: [email protected] Onshore Converter Stations Dogger Bank Development 320 kV + - 400 kV + - ~130 km ~75 km Hornsea Development To Norway 400 kV + - ~130 km Dogger Bank Hornsea To Norway To Mainland Europe Onshore AC/DC Converter Substation Offshore Node HVDC Cable To Mainland UK Mainland UK Current and Future Work During the investigations of DC/DC converters it was observed that full dynamic models of power converters are computationally intensive. This is not practical for large system studies comprising several converters. Cur- rent work is investigating reduced dynamic models of modular multi-level converters (MMC). These reduced dynamic models will be used to test potential DC node configurations and would allow sim- plified system level studies. A potential scenario is shown below, with two point to point schemes connected through a DC link with a tap through a DC/DC converter to another point to point scheme. References: [1]. D. Van Hertem, M. Ghandhari, and M. Delimar, “Technical limitations towards a SuperGrid - A European prospective,” in 2010 IEEE International Energy Conference, 2010, pp. 302–309 [2]. D. Jovcic, “Bidirectional, high-power DC transformer,” Power Delivery, IEEE Transactions on, vol. 24, no. 4, pp. 2276–2283, 2009 [3]. C. E. Sheridan, M. M. C. Merlin, and T. C. Green, “Assessment of DC / DC Converters for use in DC Nodes for Offshore Grids,” in 10th IET International Conference on AC and DC Power Transmission (ACDC 2012), 2012 [4]. C. E. Sheridan, M. M. C. Merlin, and T. C. Green, “Study of a Resonant DC / DC Converter in Alternate Discontinuous Mode,” in IEEE PES General Meeting, 2013. DC / DC Converters Given that cable technology increases by tens of kilovolts at each iteration of the technology, this implies that adjacent projects may not be built at the same DC voltage level. A DC/DC converter would allow DC sys- tems at different operating voltages to be interconnected. They could also enable different technologies to be interconnected, and provide fault blocking capabilities. Several DC/DC converters were studied and a resonant topology was studied in detail, shown in Figure 3. The advantages of this resonant converter are that it uses thyristor devices and can provide DC fault blocking capa- bilities [2]. It was seen that this converter is best suited for low conversion ratio applications, like in a MTDC grid, given that losses increase with conversion ratio [3]. A new mode of operation was developed for this converter, termed the alternate discontinuous mode (ADM) [4]. Using ADM the converter can provide fault blocking for all scenarios. The energy is transferred through a bridge of the converter and stored for a short period of time in the intermediary capacitor, the energy is then transferred to the opposite bridge. This means that the two bridges are never connected to each other at the same time. Example waveforms can be seen below in Figure 6. There are several proposals as to how this grid will arranged, one thing each proposal has in common is that several HVDC links connect at a single point, or node. This research is proposing the use of DC technology to in- terconnect these links. There are several advantages to this: • Simpler power conversion chain • No need for reactive power compensation • More flexible control of real and reactive power • Fewer cables (compared with AC) Also given the significant distances between wind farms, DC connection would be more cost effective than AC. DC nodes would make use of power electronic based converters. Research will outline potential equipment and technology for use in a DC node, followed by investigating optimum arrangements for these nodes. DC Nodes Supervisor: Prof. Tim C. Green V1 T1 T2 T1 T2 T5 T5 T6 T6 Vc I2 V2 I1 2Cr L1/2 T3 T4 T4 T3 T7 T8 T7 T8 2Cr L1/2 L2/2 L2/2 Fig 3: Circuit diagram shows that the converter has a symmetrical design Fig 6: Capacitor voltage waveform shows the individual circuit charging stages, the current waveforms show the transfer of charge from one bridge to the other (phase control thyris- tor, Case C) 1. 5 1.5005 1.501 1.5015 1.502 1.5025 1.503 1.5035 1.504 1.5045 1.505 -800 -600 -400 -200 0 200 400 600 800 Votlage (kV) Time (s) 1.5 1.5005 1.501 1.5015 1.502 1.5025 1.503 1.5035 1.504 1.5045 1.505 -2000 0 2000 4000 6000 8000 10000 12000 Current (A) Time (s) V c I 1 I 2 Fig 1: Several North Sea Grid, and SuperGrid, arrangements proposed by different organisations Onshore Converter Stations Dogger Bank Development 320 kV + - 400 kV + - ~130 km ~75 km Hornsea Development To Norway (A) (B) Fig 4: Basic LC circuit and waveforms ωt ωt V 0 V 2V -V 0 π π/2 Î V C I Vc V C L I Vc 2Cr 2Cr L2/2 L2/2 I2 V2 L1/2 L1/2 I1 V1 Energy Transfer Energy Transfer Fig 5: Charge is transferred to Cr, and is stored for a short period of time before being transferred to the output Onshore Converter Stations Creyke Beck A 320 kV + - ~75 km Hornsea Development ~130 km Dogger Bank Development Creyke Beck B 320 kV + - 400 kV + - ~130 km Fig 7: Potential scenario Friends of the SuperGrid European Commission EWEA scotland.gov.uk
Transcript of DC Nodes for an Offshore SuperGrid
Caitríona E. Sheridan
DC Nodes for an Offshore SuperGrid
MotivationAn offshore grid in the North Sea has the potential to increase the penetration of renewable energy in Europe
and to improve international energy transfer between European countries. It has been long acknowledged that this could be a precursor to a pan European SuperGrid [1]. It has been well established that HVDC technology will be used to transport this energy, given its suitability to long distance submarine cable transmission compared with HVAC. Furthermore voltage source converters (VSC) is more suitable for realising multi-terminal DC (MTDC) grids as power flow reversal is achieved by reversing current polarity as opposed to voltage polarity with classic HVDC technology. However there are several challenges with creating a MTDC grid.
This research focuses on the technical aspects of how a MTDC grid could be developed using nodes to route power flow. It will investigate several technologies for use in such a grid and determine potential optimum ar-rangements for these nodes. The case study system will be the Dogger Bank Round 3 project.
Fig 2: Potential MTDC grid and node arrangements for the North SeaA) AC arrangementB) DC arrangement
This work is funded by National Grid UK and Ove Arup & Partners
Contact: [email protected]
OnshoreConverterStations
Dogger Bank Development
320 kV+-
400 kV+-
~130 km
~75 km
Hornsea Development
To Norway
400 kV+-
~130 km
Dogger Bank
Hornsea
To Norway
To Mainland Europe
Onshore AC/DC Converter Substation
Offshore Node
HVDC Cable
To Mainland UK
Mainland UK
Current and Future WorkDuring the investigations of DC/DC converters it was observed that full dynamic models of power converters
are computationally intensive. This is not practical for large system studies comprising several converters. Cur-rent work is investigating reduced dynamic models of modular multi-level converters (MMC).
These reduced dynamic models will be used to test potential DC node configurations and would allow sim-plified system level studies. A potential scenario is shown below, with two point to point schemes connected through a DC link with a tap through a DC/DC converter to another point to point scheme.
References:[1]. D. Van Hertem, M. Ghandhari, and M. Delimar, “Technical limitations towards a SuperGrid - A European prospective,” in 2010 IEEE International Energy Conference, 2010, pp. 302–309[2]. D. Jovcic, “Bidirectional, high-power DC transformer,” Power Delivery, IEEE Transactions on, vol. 24, no. 4, pp. 2276–2283, 2009[3]. C. E. Sheridan, M. M. C. Merlin, and T. C. Green, “Assessment of DC / DC Converters for use in DC Nodes for Offshore Grids,” in 10th IET International Conference on AC and DC Power Transmission (ACDC 2012), 2012[4]. C. E. Sheridan, M. M. C. Merlin, and T. C. Green, “Study of a Resonant DC / DC Converter in Alternate Discontinuous Mode,” in IEEE PES General Meeting, 2013.
DC/DC Converters Given that cable technology increases by tens of kilovolts at each iteration of the technology, this implies
that adjacent projects may not be built at the same DC voltage level. A DC/DC converter would allow DC sys-tems at different operating voltages to be interconnected. They could also enable different technologies to be interconnected, and provide fault blocking capabilities.
Several DC/DC converters were studied and a resonant topology was studied in detail, shown in Figure 3. The advantages of this resonant converter are that it uses thyristor devices and can provide DC fault blocking capa-bilities [2]. It was seen that this converter is best suited for low conversion ratio applications, like in a MTDC grid, given that losses increase with conversion ratio [3].
A new mode of operation was developed for this converter, termed the alternate discontinuous mode (ADM) [4]. Using ADM the converter can provide fault blocking for all scenarios. The energy is transferred through a bridge of the converter and stored for a short period of time in the intermediary capacitor, the energy is then transferred to the opposite bridge. This means that the two bridges are never connected to each other at the same time. Example waveforms can be seen below in Figure 6.There are several proposals as to how this grid will arranged, one thing each proposal has in common is that
several HVDC links connect at a single point, or node. This research is proposing the use of DC technology to in-terconnect these links. There are several advantages to this:• Simpler power conversion chain• No need for reactive power compensation• More flexible control of real and reactive power• Fewer cables (compared with AC)
Also given the significant distances between wind farms, DC connection would be more cost effective than AC. DC nodes would make use of power electronic based converters. Research will outline potential equipment and technology for use in a DC node, followed by investigating optimum arrangements for these nodes.
DC Nodes
Supervisor: Prof. Tim C. Green
V1
T1
T2 T1
T2 T5
T5T6
T6
Vc
I2
V2
I1
2Cr
L1/2
T3
T4
T4
T3 T7
T8 T7
T8
2Cr
L1/2
L2/2
L2/2
Fig 3: Circuit diagram shows that the converter has a symmetrical design
Fig 6: Capacitor voltage waveform shows the individual circuit charging stages, the current waveforms show the transfer of charge from one bridge to the other (phase control thyris-tor, Case C)
1. 5 1.5005 1.501 1.5015 1.502 1.5025 1.503 1.5035 1.504 1.5045 1.505−800
−600
−400
−200
0
200
400
600
800
Votl
age
(kV)
Time (s)
1.5 1.5005 1.501 1.5015 1.502 1.5025 1.503 1.5035 1.504 1.5045 1.505−2000
0
2000
4000
6000
8000
10000
12000
Cur
rent
(A)
Time (s)
Vc
I1
I2
Fig 1: Several North Sea Grid, and SuperGrid, a r r a n g e m e n t s proposed by different organisations
OnshoreConverterStations
Dogger Bank Development320 kV+-
400 kV+-
~130 km
~75 km
Hornsea Development
To Norway
(A)
(B)
Fig 4: Basic LC circuit and waveforms
ωt
ωt
V0
V
2V -V0
ππ/2
Î
VC
I
VcV C
L IVc
2Cr2Cr
L2/2
L2/2I2 V2
L1/2
L1/2I1
V1
Energy Transfer Energy Transfer
Fig 5: Charge is transferred to Cr, and isstored for a short period of time beforebeing transferred to the output
OnshoreConverterStations
Creyke Beck A
320 kV+-
~75 km
Hornsea Development
~130 km
Dogger Bank DevelopmentCreyke Beck B
320 kV+-
400 kV+-
~130 km
Fig 7: Potential scenario
Friends of the SuperGrid
European CommissionEWEA
scotland.gov.uk
DC Nodes for an Offshore Grid Poster Synopsis:
As more generation is commissioned and built offshore, it is likely that this generation will be interconnected to several European countries using a high voltage DC (HVDC) grid. There are many different proposals as to how this grid will be arranged, one element they all have in common is that several HVDC links meet at a single point. This research proposes the use of DC technology to interconnect these links, using DC nodes. These nodes can be used to apply additional power flow control and provide fault blocking for a multi-terminal DC grid. A DC node cannot be thought of as a single point, as it could encompass a large geographical area. Before a potential node configuration can be proposed, the technology that could be used must first be studied. This poster outlines the work carried out to date on DC/DC converters and AC/DC converters, and outlines future work.
