Stability analysis of power network using 600kV, 800kV, and 1000kV HVDC

Stable power network

An attempt to have a more stable power network in order to meet the ever-increasing energy demand and totally eradicate power failure in South Africa brings about the construction of new power station like Medupi, coal 2 and coal 3 power station, which will give a gross nominal capacity of 30 GW by 2030. Integration of these power stations into the grid could be either through High Voltage Alternating Current (HVAC) or High Voltage Direct Current (HVDC) transmission line.

Transmitting using AC lines has concerns such as, the thermal limit, corona loss, skin effect, Ferranti effect, economics of transmission, reliability, cascading problem, and finally the network Right of Way (ROW).

Now the questions that this research seeks to answer are; can the existing AC transmission line be upgraded to a DC line, what benefit does it offer as compared to the existing AC line with respect to the power output and losses?

If the transmission line voltage is increased to 600kV and then to 800kV, what effect does it has on the system stability of the entire networks compared to HVAC line of equivalent rating.

Transmission of high voltages exceeding the rated installed capacity also causes electricity to flashover, rather than travel along the line. It also causes corona discharge, which leads to systems instability.

Is it possible to develop a strategy for upgrading the transmission line by either increasing the voltage or current or even both or still have a well-stabilized transmission network and solve all these constraints? To what extent is HVDC immune to fault current, that is, how fast does it takes to clear a fault, and the systems restored to its original state?

Eskom has only one HVDC network installation, transmitting 1920MW power at a voltage level of ±533kV from hydroelectric generation station located at Cahora-Bassa Dam in Mozambique to Johannesburg.

How will an HVDC line be incorporated into Eskom power network and stabilizes its performance, and what converter technology bring about highest power output, enhanced stability, immunity to commutation failure and finally reduce harmonic content?

This research investigation aimed to achieve the following:

1. Investigating the effect of grid planning applications such as increasing in transmission lines loading capacity near their steady state, short time and dynamic limits on Eskom power network’s stability and performance.
2. Upgrading better transmission lines either by increasing the voltage, current capacity or both.
3. Evaluating the technical performance and the stability of Eskom power network when fed with 600, 800, and 1000kV HVDC along Limpopo transmission network to Eskom Eastern grid.

In order to achieve the aim of this project, an Eskom Eastern transmission network was modelled using Digital Simulation and Electrical Network Calculation Program (DIgSILENT) PowerFactory software. A technical examination of its performance was then carried out on the network.

An extensive literature survey was carried out on:

• HVAC and HVDC transmission networks; advantages and disadvantages
• Recent HVDC transmission network technology and mode of operation.
• Line performance assessment of HVAC and HVDC lines.
• Stability and technical performance of HVDC transmission line on HVAC networks.

Features of HVDC delivery system

The invention of modern semiconductor devices for HVDC converter made HVDC transmission systems to be widely adopted for different application such as electrical and electronic power distribution systems, micro grids and super grids for renewable energy integration.

Few reasons are listed below to why HVDC systems should be more encourage and adopted in electric energy transmission.

Power carrying capability

For HVAC and HVDC system with an equal current rating of I_ac = I_dc, same insulation length in each conductor, as well as the same number of conductors, double circuits of three-phase HVAC system with six conductors, will generate three-bipolar HVDC system.

More so, the two conductor of DC transmission tends to reduce the transmission losses to about two third of the comparable AC systems. This ability of HVDC systems to operate at a voltage higher than AC systems brings about the high power capability, with voltage rating up to 1600kV (±800kV) in service and ±11000kV ongoing project in China.

The first highest known commercialized AC lines was 1200kV on the line connecting Russia to Kazakhstan, which came into operation in 1988 and later, was dismantled by 1996 and then operated at 500kV.

China is now the first country to introduce 1000kVAC transmission line to be operated at full rated voltage. The 650km line has a power carrying capacity of 5000MW.