4.1. Conclusions
HVDC transmission technology is increasingly interested and widely applied in the world. Compared with HVAC transmission technology, HVDC technology has several advantages when applying long distance transmission and large transmission capacity.
The outstanding advantages include: the investment cost for HVDC is lower than HVAC with the same voltage; low loss; There is no reactive power generated on the line so there is no restriction on the transmission length; ... HVDC is applied a lot in electrical systems linking countries and regions; submarine cable lines or areas where AC is difficult to build.
The size of Vietnam's power system has grown at an average of 11% per year in the last 10 years and is expected to continue to grow at a high level in the next 10-25 years. The electricity system still needs to be invested and constructed to provide enough electricity for production and social life. Corresponding to the development of power sources, the transmission grid system is also invested accordingly to ensure smooth, safe, reliable and stable power transmission. Due to the long and narrow topography from the North to the South, the power sources are distributed far from the load center, so in the transmission expansion plans with large capacity (over 1,000 MW), the use of HVDC technology should be considered compared to the traditional HVAC transmission option.
This study is one of the supplementary studies for the design of the transmission grid of the National Power Development Plan for the period 2021-2030, taking into account 2045 (PDP 8). The project has calculated the economic and technical comparison of 84 HVDC and HVAC transmission scenarios corresponding to different transmission power levels (1000 MW to 6000 MW) and different distances (300 km to 2000 km) in the context of Vietnam.
After analyzing and calculating the investment cost for the 525 kV HVDC system (LCC technology) and the 500 kV HVAC, power loss and current cost, the conclusions were made as follows:
Regarding the investment capital for construction of transmission system:
- At the transmission distance of 300-400 km, the HVDC option of 525 kV and HVAC 500 kV have the same investment capital.
- The larger the transmission capacity scale, the greater the distance, the more HVDC shows the advantage of investment capital. At a distance of 1000 km,
the investment capital for HVDC transmission systems is only 70% -80%
compared to HVAC. At distance of 2000 km, the rate is only 62-70%.
About transmission loss:
- Transmission losses on the HVDC system are basically lower than the HVAC.
However, this difference is not large, only about 0.2% to 2.1%.
- At the transmission distance of less than 600 km, the difference of HVAC and HVDC loss is negligible, from 0.2% to 0.6%.
- In some cases, for long-distance transmission lines (over 1000 km), the loss on the 525 kV HVDC system is higher than the HVAC 500 kV from 0.1% to 0.9%
(if series capacitor are installed, reasonable reactor compensation installed).
Regarding the NPV cost:
When considering investment capital, power loss and 40-year project life, the NPV costs of HVDC and HVAC technology are similar in the transmission distance of 300-400 km. For transmission distances of 600 km or more, HVDC technology has significantly lower NPV costs, from 11% to 25% compared to HVAC technology.
Potential HVDC transmission projects:
The study also analyzes the outputs of the long-term power development program for 11 different scenarios. Based on the capacity of transmission capacity that needs to be strengthened on regional links, the HVDC transmission option may be considered for comparison and selection at some potential locations:
By 2030:
Source Scenarios: 0A, 1A_RE, 1B_RE, 5B, 6B: Consider the option of transmitting 2000 MW from the Central Highlands to the South on a distance of 400-600 km.
According to calculations, the NPV cost of the HVDC +/- 525 kV option may be 25%
lower than the HVAC 500 kV.
Source scenario: 2A_RE, 2B_RE, 3A_RE, 3B_RE, 4A_CO2, 4B_CO2, considering the option of transmitting 2000 MW from South Central to the North over a distance of 1500 km. According to calculations, the NPV cost of the HVDC +/- 525 kV option is about 15.5% lower than the HVAC 500 kV.
By 2045:
Source scenario: 2A_RE and 5B_Nonewcal, it is necessary to enhance the central-central power transmission line to the North with the scale of ~ 2000 MW over a
distance of 600 km. The NPV cost of the HVDC +/- 525 kV system in this case may be approximately 25% lower than the HVAC 500 kV.
In all source scenarios, by 2045, the source from the Highland need to be transmitted to the South to increase by about 3 GW to 5 GW over the transmission distance from 400 km to 600 km. Constructing HVDC system will be lower than HVAC 500 kV by about 14% - 20%.
The need to increase transmission capacity from the South Central region to the South increases in all scenarios. Calculation results in 2045 show that the transmission capacity on the interface needs to increase from about 4 GW to 16 GW depending on the source development scenario. According to calculations, for the 300-400 km transmission distance on this interface, HVDC technology will be significantly more efficient than the 500 kV HVAC starting from a transmission capacity of 3000 MW or more (NPV cost lower from 9% to 11%).
The transmission capacity over 1500 km from South Central Coast to the North is also high in many scenarios, from 2 GW to 9 GW (KB5B_Nonewcoal). For a distance of 1500 km and a transmission capacity of 2-6 GW, the NPV cost of HVDC +/- 525 kV technology is 16% to 22% lower than that of HVAC 500 kV.
4.2. Recommendations
The choice of HVDC or HVAC transmission technology depends greatly on RE development goals and the philosophy of power development in Vietnam. HVDC technology is a new technology in Vietnam, it takes a lot of time to research and develop, develop standard systems and specialized regulations. Therefore, the decision to use HVDC technology should be made in a stable development roadmap for pursuing the government's targets of RE development and consistency in power development strategy. Lessons of sudden changes in policies for new technologies, such as policies for developing nuclear power technologies in Vietnam, should be avoided, leading to a waste of training, research and development resources over many years.
This research is only the first step in determining the applicability of HVDC in developing backbone transmission network in Vietnam. Given the posetive prosepcts for HVDC in this first step study, it is recommended to conduct more in-depth studies are needed to clarify the feasibility of HVDC technology, the choice voltage level is +/- 525 kV or may be lower (like +/- 320 kV) or higher (like + / - 600 kV, +/- 800 kV).
LCC technology or VSC technology also needs to be analyzed at some specific transmission projects.
The technical in-depth aspects also need to be further investigated such as the effect of the HVDC system on the stability of the power system, short-circuit current, voltage, power supply reliability, ...