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Reference Number NIA_SPT_1309
Title Low Frequency Electricity Transmission Technology Evaluation
Status Completed
Energy Categories OTHER POWER and STORAGE TECHNOLOGIES(Electricity transmission and distribution) 100%;
Research Types Applied Research and Development 100%
Science and Technology Fields ENGINEERING AND TECHNOLOGY (Electrical and Electronic Engineering) 100%
UKERC Cross Cutting Characterisation Not Cross-cutting 100%
Principal Investigator Project Contact
No email address given
SP Energy Networks
Award Type Network Innovation Allowance
Funding Source ENA Smarter Networks
Start Date 01 December 2013
End Date 01 January 2016
Duration 27 months
Total Grant Value £175,000
Industrial Sectors Power
Region Scotland
Programme Network Innovation Allowance
Investigators Principal Investigator Project Contact , SP Energy Networks (100.000%)
Web Site
Objectives To comprehensively literature review, model and evaluate the low frequency ac transmission technology and its potential in transmission business A report with tangible scientific evidence, and clear recommendation on a further demonstration project
Abstract The main attraction of VSC-HVDC, as compared to conventional HVDC using line-commutated converters, is the relatively small space requirement for the filters which could be difficult to accommodate offshore (3,4). But a multi-terminal DC network requires DC circuit breakers which are not economically available in spite of escalated efforts in the last few years (5). Effort is also being made to develop VSCs which can block/limit the current infeed upon the occurrence of pole-pole faults on the DC side (6). This would be useful but cannot eliminate the need of DC circuit breakers and further challenges are yet to be addressed to provide the selectivity in fault clearance. Point-to-point, two-terminal VSC-HVDC systems have been commissioned. Faults on the DC side can be cleared from the AC sides using AC circuit breakers on natural arc-extinction as the fault current crosses zero. In a multi-terminal HVDC system, this method is not applicable and, without DC circuit breakers, the entire system needs to be shut-down and then re-started later. Future DC circuit breakers may also need to handle the large discharging current from the capacitors inherent in the VSCs. Another major issue is the reliability of high voltage DC cables based on XLPE technology which has made VSC-HVDC economically competitive. XLPE cables are subject to the adverse effect of space charge accumulation and the water treeing problem associated with it and the humidity that can penetrate into the cable insulation structure (7). There is strong evidence that such a problem is much exacerbated in the case of DC as compared to AC , while without a complicated switchgear arrangement and coordination between terminals the DC voltage polarity of a VSC-HVDC system cannot be easily reversed. There is only very limited experience regarding the operational reliability of XLPE HVDC cables, whilst condition monitoring and preventive maintenance in the offshore environment will be difficult (8). The reason that prevents a standard AC network from being used in this target offshore application is the large charging/discharging reactive power associated with the subsea cable, which depends on the transmission distance, power level and hence the required voltage. For instance the charging reactive power for a 100 km, 50 Hz and 400 kV cable reaches 1500 MVAr, exceeding the natural power for such a three-phase cable circuit (9). This proposal envisages that the charging reactive power can be reduced by adopting an operational frequency much lower than 50 Hz. Another claimed advantage of AC against AC is that fewer conductors are required for the same power level (10). The difference is however less significant as the cost of deployment, rather than the additional material and manufacturing cost, will dominate. Adopting a lower AC frequency (e.g. in the range of 0. 5 to 5 Hz, to limit the charging reactive power to 1~10%) but still high voltage (e.g. 400 kV), would allow the use of AC circuit breakers and the cable will work under alternating electric field without accelerated space charge accumulation. However some challenges have to be faced in order to make this a reality. The utmost challenge is that the size of the low frequency transformers would at first glance be prohibitively large (11). This is addressed in the proposal next while other secondary challenges will be described later.Note : Project Documents may be available via the ENA Smarter Networks Portal using the Website link above
Publications (none)
Final Report (none)
Added to Database 31/08/18