Appendices
Chapter 3: Research Design and Methodology
3.1 Introduction
3.2 Development of the Hypothesis
3.2.1 Existing Infrastructure: The South African (Eskom) national grid is composed primarily of single circuit 275 kV and 400 kV transmission lines with a small but growing population of 765 kV transmission lines. The phases are arranged in a horizontal configuration with two overhead earthwires for lightning protection. It is known that the current carrying capability of the installed phase conductors far exceed the surge impedance loading level and this gap between the two parameters represents the opportunity for promoting higher power transfers using HYDC technology.
The South African National Grid is planned and designed to an (N-I) contingency capability;
thus affording one circuit to be made available for upgrade and conversion to HYDe. The upgrade and conversion work can be conducted whilst the HYAC circuits continue to operate;
once the HYDC is fully commissioned; the switchover can occur. It is further noted that a flip- flop switch over back to HY AC is possible and will form part of the design feature for this proposal. Such flip-flop capability will assist during periods of planned maintenance of the HYDC scheme, but admittedly, at a lower load transfer capability. On introduction of the HYDC design, a further (N-I) contingency capability will be introduced by monopolar operation using the metallic earth return.
3.2.2 Limiting Conditions: Clearly, it is not proposed that all HYAC circuits become candidates for upgrade and conversion to HYDe. The introductory study and literature review have shown that the initial boundary conditions for admittance of a line for upgrade and conversion will be based on the magnitude of the unused power transfer gap (UPTG) between SIL and CCCe. This conclusion was published in two peer reviewed international conferences and accepted technically [30, 31]. In summary, we have:
Unused power transfer gap is small (equal to SIL) - do nothing
Unused power transfer gap is appreciable and of magnitude I to 2 times SIL - employ FACTS technology
Unused power transfer gap is large and of magnitude 3 to 4 times SIL - ideal candidate for upgrade to HYDC
At this stage, we introduce from Hingorani and Gyugyi [32] and their perspective on HYDC and FACTS. "Both HYDC and FACTS are complementary power electronic based technologies;
having different value added contributions to the AC network. HVDC is not a grid network technology; but uses transmission to connect two nodes of an AC grid network. Here transmission could be a underground or submarine cables, overhead transmission lines or just a busbar as in a conventional back to back scheme for interconnecting different AC frequency networks. For both FACTS and HYDC application, the market potential within the AC network is on a value added basis. For both technologies, power control, voltage control and stability control exists; whereas HYDC can go onto provide lower power losses, independent frequency and control and for new lines, a lower line cost as compared to equivalent AC. Capital costs for power transfer throughput are the only key decision selector for power transfers in the MW range, favouring FACTS over HYDC; whereas in the multiple GW range and over longer distances, HYDC leads FACTS technically and economically." This perspective from Hingorani and Gyugyi [32] concurs with the proposed boundary condition.
From the proceedings of the three international conferences [30, 31, and 37] on ACDC Transmission, it was clear that the development of the HYDC solution would yield additional strategic and economic benefits. These include:
• The difficulty and high costs for acquiring new power line servitudes;
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The substantial savings that can be achieved if existing assets can be upgraded for higher power transfers;
The rising need to introduce HYDC technology to address transmission congestion and bottlenecks in existing complex networks
The rising need to introduce HYDC technology to create electrical islands for the controllability of large and complex AC networks.
To promote and enhance power system stability by using the HYDC controllers in parallel with HYAC operations
• To support the growing ancillary services energy management requirements by making available regulation using the DC controllers.
For the final limiting conditions, table 3-1 introduces the statutory occupational health and safety act regulations as defined for South Africa. Emanating from the Occupational Health and Safety Act N085 of 1993, the study notes the following final boundary conditions that would apply to the HYAC and HYDC configured transmission lines [33].
Table: 3-1: Electrical Machinery Regulations for Power Line Clearances
Maximum Minimum Minimum Minimum Minimum Minimum Minimum Voltage for safety clearance clearance clearance clearance clearance which clearance in meters in meters in meters in meters in meters
insulation in meters above above to roads, to to
is designed ground ground railway communi buildings,
and and inside and cation poles and
outside townships tramways lines structures townships
300 kV 2,35m 7,2m 7,2m 8,4m 2,9m 4,7m
rms. phase to phase
420 kV 3,2m 8,lm 8,lm 9,3m 3,8m 5,6m
rms. phase to phase
800 kV 5,5m 10,4m 10,4m 11,6m 6,lm 8,5m
rms. phase to phase
533 kV DC 3,7m 8,6m 8,6m 9,8m 4,3m 6,lm
maxImum voltage to earth
The proposed research hypothesis is presented in Figure 3-1.
Negative Polarity
Substation A Substation B
HVAC .. HVAC
HVDC HVDC
Positive Polarity "Metallic" Earth Return
Figure 3-1: Proposed Upgrade and Conversion of a three phase HVAC transmission line toHVDC
In summary, the physical transmission line remains as IS In structure for both HVAC and HYDC application; the additions being the inverter and rectifier converter stations and the change in external insulation, if required.