Innovation in Global Power - Parsons Brinckerhoff

More documents

Recommendations

Info

Transporting Power Across the Grid Typical HVDC Configurations The following are typical HVDC configurations (a dc line with connected converters is referred to as a pole): • Monopole with earth return or metallic return (with monopole designs, the earth electrode or metallic earth return conducts full current during normal operation) (Figure 2) • Bipole with earth return or metallic return (Figure 3) • Bipole without earth return or metallic return. Figure 2: Monopole with earth or metallic return. Figure 3: Bipole with earth or metallic return. In a bipole configuration, the converter stations are arranged to operate at equal but opposite line voltage so that the current in the earth return path is very small under normal operation. One of the advantages of the bipole dc configuration if there is a working earth return path or a metallic return is its 50 percent redundancy. With one pole out of service, the remaining pole operates in monopole mode and can still transmit 50 percent of the link power. Bipoles can be designed to transmit up to 75 percent of link power for short periods under loss-of-pole contingencies if the converter equipment is designed for short-time overload. Figure 4 shows some transmission tower configurations for monopole and bipole dc solutions. • Left drawing shows the simplest monopole earth return tower configuration. In this instance, the earth electrodes and the earth return operate continuously. http://www.pbworld.com/news_events/publications/network/ • Center drawing depicts a bipole configuration without a metallic return. • Right drawing depicts a bipole configuration on a single structure.The advantage of a double-circuit tower bipole configuration is the lower cost (one tower and set of foundations instead of two). Its disadvantage is its vulnerability to common mode failure; e.g., a tower collision can take out both lines and, thus, both poles instead of only one line. In Africa, where there are semi-deserts or sparsely populated areas, monopole earth return configurations may be attractive due their low costs (only one overhead conductor bundle and one converter is required at each end of the dc link). In addition, these dc monopole links may connect radial ac systems, so the requirement for dc redundancy or for (n-1) operation of the dc link becomes less necessary. In some weak African systems, conventional dc converters or line commutated converters (LCC) cannot operate as the short-circuit-to-power transfer ratios at both converters do not exceed the necessary level of three (SCR
Transporting Power Across the Grid Figure 5 depicts the South African generation and transmission system in a highly schematic way. The major generation system in South Africa, which is located in Mpumalanga, supplies Johannesburg, Pretoria, Durban, Richards Bay, Cape Town and Port Elizabeth. When and if the Medupi, Mmamabula and Mmamantswe power stations are commissioned (in the north west of South Africa and in Botswana), capacity on the Mpumalanga power stations will be released to supply large growing load in the Central Area, Eastern area and the Cape. Medupi power station is an Eskom coal-fired power station with a design capacity of 4800 MW. Mmamabula and Mmamantswe power stations are proposed IPP coal-fired power stations north of Gaborone City, in Botswana. The dc transmission will not be stranded if large generation power stations are constructed in Cape Town because it can be used to transmit power from the south to the north when large nuclear power stations come on stream in the Cape and increase the stability of the transmission link. A second option is to strengthen the networks to the Cape via a 765 kV ac system from Zeus Substation in Mpumalanga to Omega Substation near Koeberg Power Station in the Cape (Figure 6). Figure 6: Proposed ac strengthening to the Cape. 2012 765 kV Zeus-Mercury- Perseus-Gamma- Omega. http://www.pbworld.com/news_events/publications/network/ Another benefit of a dc link to the Cape, as stated above, would be its contribution to system stability, over the very long transmission distance (1400 km) between northern generation and southern generation systems. Conclusions Figure 7: Proposed dc strengthening to the Cape. 2012 765kV AC Solution followed by DC to Gamma from Zeus. DC can be a cheaper transmission alternative to ac over long transmission distances while also improving system stability and providing improved reliability due to the requirement for fewer conductor bundles and the redundancy in the dc bipole configuration. Depending on the nature of the African systems involved, cost effective monopole earth return dc options can be investigated. Where more reliable and robust transmission systems are required, bipole dc systems can be investigated. DC is a credible transmission option for strengthening the Cape in South Africa. Related Web Sites: • http://www.eepublishers.co.za/view.php?sid=5057 Figure 7 shows part of the ac system as proposed in the previous Cape strengthening option followed by a dc bipole system from Zeus Substation to Gamma Substation in the Northern Cape. A 500 kV 2000 MW bipole solution could be investigated. This solution would add 1000 MW power increments in each pole of the bipole, matching the power output of single Koeberg Power Station units. Operationally, a loss of a single pole would only disrupt the system by 1000 MW. The 1000 MW disruption could be reduced if the remaining pole is designed with a short-time overload capability. Acknowledgment: I wish to thank fellow engineers and colleagues for their assistance and contributions to this article. Paul Tuson is a degreed Transmission Studies Specialist with more than 18 years’ post graduate experience in power electrical engineering in Southern Africa, the UK, Australia, the USA and the Middle East for voltages up to 765kV. Key experience is in the areas of system analysis and financial/economic evaluation of capital projects and aggregate expansion plans. Paul specialises in complex system modelling in the areas of loadflow, fault, transient stability and dynamic stability, and he is proficient in a range of power simulation software including PSS/E, DigSilent, PowaMaster, ReticMaster, ETAP, ERACS, EDSA and CYME. Other studies he completed recently include an HVDC and FACTS interconnector study in Namibia for NamPower (350kV dc), a 25- year least cost transmission master plan for Tanzania (330kV and 220kV), and generation integration studies for various utilities in the UK, South Africa and Tanzania. 65 PB Network #68 / August 2008
Page 1 and 2:
A technical journal by Parsons Brin
Page 3 and 4:
http://www.pbworld.com/news_events/
Page 5 and 6:
Thermal - Achieving New Efficiencie
Page 7 and 8:
Page 9 and 10:
Page 11 and 12:
http://www.pbworld.com/news_events/
Page 13 and 14: Thermal - Achieving New Efficiencie
Page 21 and 22: GENERATION: Hydropower - New Techno
Page 23 and 24: Hydropower - New Technologies, New
Page 25 and 26: http://www.pbworld.com/news_events/
Page 47 and 48: Renewables - The Risks, Concerns an
Page 59 and 60: Transporting Power Across the Grid
Page 63: Transporting Power Across the Grid
Page 69 and 70: Distributing Power to Users Researc
Page 71 and 72: Distributing Power to Users The fin
Page 73 and 74: Distributing Power to Users initial
Page 75 and 76: Distributing Power to Users A Surve
Page 79 and 80: Distributing Power to Users http://
Page 81 and 82: Planning and the Role of Regulators
Page 91 and 92: DEPARTMENTS PB’s power specialist
Page 93 and 94: Networking • What CPUs and PLCs w
Page 97 and 98: Networking 8. Dash Style. You can u
Page 101 and 102: Going Green: Walking the Walk!! By
show all

Innovation in Global Power - Parsons Brinckerhoff

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?