Project PS-A: Multi-node DC Transmission Networks
Manager: Tim Green
Investigator: Tim Green
Research Staff: Dr Michael Merlin (Research Associate), Thomas Lüth (PhD Studentship)
Collaborators: Alstom Grid, National Grid
Start date: 01/06/2012 (RA) and 01/10/2010 (PhD)
Linked Projects: PS-D and
UT-D
Summary. The UK government’s ambitions for carbon emissions reductions in the energy sector are radical and it is clear that the electricity sector will need to be almost completely decarbonised over the next two decades. There is no single solution that is credible and, instead, a variety of measures will be needed. A substantial fraction of electrical energy will have to be sourced from marine renewables, principally off-shore wind and tidal flow, since the UK is particularly well endowed with these resources. The problem is that the north-south transmission corridors in the UK do not have sufficient capacity to carry the additional generation that is foreseen in the north under this scenario. One example is the Scottish-English interconnection which already runs close to it maximum capacity of around 3GW (set by a stability limit) and cannot carry the expected 20GW of additional wind generation in Scotland. With new-build overhead lines virtually ruled out because of planning permission problems, the UK will need long-distance underground or sub-sea cable connections. For distances of more than about 50 km such cables will be operated on DC not AC. The amount of new transmission infrastructure and the need to provide some security in transmission implies an interconnected (meshed) DC network of a type not previously attempted. To date, HVDC transmission has been point-to-point links with some examples of three-node links recently appearing.
Current Status. Merlin has continued his work on control of HVDC converter stations covering both normal and abnormal operating conditions that were the focus of his PhD which was obtained in July 2013. Thomas Lüth took up a PhD studentship from October 2010 and has been asked to consider circuit topologies and control for DC/DC conversion. In parallel with this we have been constructing an experimental test rig.
Modular forms of AC/DC power converter show some significant benefits in reducing power loss compared to circuits with “valves” composed of many devices in a simple series connection. This has been pioneer commercially by Siemens. However, a 1000A, ±300 kV system may have more than 1,000 modules and therefore 1,000 capacitors whose energy balance must be controlled through both normal and abnormal operating conditions. Dr Soto has been exploring the control options available for how energy is directed to different groups of modules and verifying controller designs. The problem is complicated by the limited bandwidth available in which to achieve control of several objectives. This precludes using a control hierarchy based on inner loops being faster than outer loops. A parallel piece of work has been undertaken with Alstom on the corresponding issues in the “alternate arm converter” proposed by Alstom.
Dr Merlin has also explored the design and control of the “Alternate Arm Converter” (AAC). This converter topology is a hybrid version between the 2-level VSC and the Modular Multi-level Converter (MMC) and offers both low harmonic distortion of the AC and DC-side fault response at an acceptable power losses level in opposition to previous VSC topologies. Alstom has now placed a consultancy contract with us through Imperial Consultants to examine all the design trade-offs in more detail. Besides this strand of research, Dr Merlin has also worked in collaboration with fellow PhD students (1) on overrating of semiconductor devices in order to understand the controllability limits of HVDC converters and (2) on the study of DC-fault responsive topologies in a multi-terminal network environment and the possible fault response strategies.
There were two reasons for examining DC/DC converters. One is the desire to keep all of the offshore system DC and therefore a step up from wind turbine output voltages to HVDC link voltages. The other is the desire to “tap off” small amounts of power from HVDC links in north Africa to serve small communities. The arguments for modular converters (rather than simple series device connection) are now gaining favour in AC/DC converters, as a result of which Mr Lüth was asked to examine DC/DC counterparts. He specifically looked at front-to-front arrangement of known multi-level AC/DC converters and developed control schemes specific to these class of converter. He also generated several candidate converter arrangements and has analysed these and produced some simple simulation models. One topology emerged as preferable but it also became apparent that using a high-frequency transformer is still beneficial in reducing semiconductor ratings even in modular circuits. He is now constructing more elaborate simulation models so as to provide some verification of the analysis of device ratings and power losses.
There are two aspects of experimental work that we wish to undertake and are building up test rigs in the continuation of the efforts on the FlexNet project. To verify system-level of power flows in an HVDC network, we have constructed a physical model of a four-terminal network in an “H” configuration. This represents perhaps two offshore wind farms and two onshore connections with one cross-link for redundancy reasons. The circuit consists of 10 pi-shaped LCL sections for each of the main routes rated at 10A and ±350 V. The impedances have been chosen to match the frequency response of real cables up to about 800 Hz and give propagation delays to match 100 km lines. Four of our existing AC/DC power converters have been modified to work with is cable system. The test rig has been tried out with a basic control scheme and basic transmission scenarios have been tested. The second aspect is to build modular multi-level converters, especially those of the type which can block DC-side faults. We had planned to get the equipment and control platform manufactured by Triphase but this was not possible so we are building this rig from scratch and buying a control platform from Opal-RT. The hardware built is making good progress, despite the high complexity of the multilevel converters requiring construction, and full commissioning is expected to take place within a few months.
Publications.
[MG14]
M.M.C. Merlin, T.C. Green,
Cell Capacitor Sizing in Multilevel Convertors: case of the MMC and AAC,
IEEE Transactions on Power Electronics, Special Issue on Modular ulti-level Converters,
to appear
[LMGHB14]
T. Luth, M. M. C. Merlin, T. C. Green, F. Hassan, C.D. Barker,
High Frequency Operation of a DC/AC/DC System for HVDC Applications,
IEEE Transaction on Power Electronics, Special Issue on High-Frequency-Link Power-Conversion Systems, Volume 29, Issue 8, pp4107-4115 ,
2014
[MGMTCCH14]
M.M.C. Merlin, T.C. Green, P.D. Mitcheson, D. Trainer, D. Critchley, R. Crookes and F. Hassan,
The Alternate Arm Converter: a new Hybrid Multi-level Converter with DC Fault Blocking Capability,
Power Delivery, IEEE Transaction on, Special Issue on HVDC Systems and Technologies, Volume 29, Issue 1, p. 310-317,
Jan, 2014
[JMMG13]
P.D. Judge, M.M.C. Merlin, P.D. Mitcheson, T.C. Green,
Power Loss and Thermal Characterization of IGBT Modules in the Alternate Arm Converter,
ECCE Denver,
2013
[CGM13]
P. Clemow, T.C. Green, M.M.C. Merlin,
Partial Power Operation of Multi-level Modular Converters under Subsystem Faults,
Power & Energy Society General Meeting, Vancouver,
2013
[SMG13]
C. Sheridan, M.M.C. Merlin, T.C. Green,
Study of a Resonant DC/DC Converter,
Power & Energy Society General Meeting, Vancouver,
2013
[LMGBHCCTD13]
T. Lüth, M.M.C. Merlin, T.C. Green, C. D. Barker, F. Hassan, R.W. Critchley, R.W. Crookes, D. Trainer, K. Dyke,
Choice of AC Operating Voltage in HV DC/AC/DC System,
Power & Energy Society General Meeting, Vancouver,
2013
[LMGBHCCD12]
T. Lüth, M.M.C. Merlin, T.C. Green, C.D. Barker, F. Hassan, R.W. Critchley, R.W. Crookes, K. Dyke,
Performance of a DC/AC/DC VSC System to Interconnect HVDC Systems,
AC and DC Power Transmission, 2012 ACDC 10th IET International Conference,
Oct. 2012
[SMG12]
C.E. Sheridan, M.M.C. Merlin and T.C. Green,
Assessment of DC/DC Converters for use in DC Nodes for Offshore Grids,
AC and DC Power Transmission, 2012 ACDC 10th IET International Conference,
Oct. 2012
[MGMTCC10]
M.M.C. Merlin, T.C. Green, P.D. Mitcheson, D. Trainer, D. Critchley, and R. Crookes,
A new hybrid multi-level voltage-source converter with DC fault blocking capability,
AC and DC Power Transmission, 2010 ACDC 9th IET International Conference,
Oct. 2010
