How East-West Power Corridor Can Extend Time For Solar-PV And Wind Power Generation

Embracing innovative solutions like a dedicated east-west HVDC corridor for combined solar-PV and wind power transmission can pave the way for a more sustainable energy future, blending efficiency with environmental stewardship

The energy mix of our electric grid is changing very fast. Every year, renewable energy is being added to combat global warming. Solar-PV and wind power are the two major renewable sources. However, both these sources are highly fluctuating in nature. Hence, as their share in the grid increases, the chances of the grid becoming unstable are higher. Therefore, extra measures must be taken to keep the grid stable. Table 1 shows the energy mix.

Hybrid (solar-PV+wind) farms are being established, as solar power generation happens during the day, while wind power is mostly available at night. Such hybrid farms offer a steady output throughout 24 hours. There are plans to establish grid-scale batteries which store energy whenever there is excess generation. The stored energy is used when the demand is more than the generation. Here, we propose a more elaborate scheme, which combines different geographical locations and time zones for establishing solar-PV and wind farms. These farms are interconnected through high voltage DC (HVDC) transmission lines. Such a power corridor offers multiple benefits.

Why East-West

Our country lies between longitudes 68° 07’ E (Gujarat) and 97° 25’ E (Arunachal Pradesh), with an angle difference of 29° 18’. The rotation time of Earth per degree longitude is approximately 4 minutes.

So, rotation time of Earth per degree longitude=24 hrs x 60 min/360=4 minutes.

And the time taken to cover 29° 18’ is given by (29° x 4)+((18’/60) x 4)=117.2 minutes.

Thus, the time difference between sunrise in the east to sunrise in the west is almost two hours. This means a solar plant in the east starts generating power two hours before the plant in the west. Similarly, in the evening, the solar power plant continues to generate power for two hours after the plant in the east has stopped its generation. This is shown in Fig. 1. The west-side power generation (green) is delayed by two hours with respect to the east (blue). The combined power is shown in red. By having two such plants, we can extend the generation of solar power for an additional two hours. To achieve this, a bidirectional transmission line between these two plants is necessary.

With such vast distance between these two locations, wind patterns will also differ. Hence, it is possible that when wind is strong in one location, the other location may have less wind, and vice versa. This difference is useful for achieving uniform power generation from windmills. Thus, the combined effect of solar and wind generation at two geographically different locations will help in generating steadier power for 24 hours.

Why HVDC

The distance between the east and west extreme points is 2933km. For such a long distance, only HVDC lines are suitable. HVDC connects two terminal stations. At these stations, AC is converted to DC, then transmitted over the line, received, and converted back to AC. The advantages of HVDC transmission include:

1. For very long distances (beyond breakeven distance), it transmits power very economically

2. Reduced power losses due to the absence of reactive power, absence of skin effect, and no dielectric losses

3. Thinner and cheaper conductors. Only two conductors are required, unlike three in HVAC lines, hence a narrower right of way, reducing land acquisition requirements

4. Full control over power flow

5. Easily integrates with smart grid

A major limitation of HVDC is that it transmits bulk power between two terminals only. And, to cover such a long distance, the initial investment required is much higher. To keep the initial investment on the lower side and to maximise utilisation, consider these points:

1. The proposed HVDC line should be installed on a fixed latitude throughout the length, without any deviation (parallel to the equator). Such a line will be the shortest one and ensure the maximum time difference between the two stations.

2. The two terminals of this corridor should have the potential to set up solar and wind farms.

3. The generation cost of solar-PV and wind power is decreasing. Hence, this power should be utilised to avoid the use of ‘peaker’ power plants which use oil, and whose generation cost is much higher. This criterion should be followed while deciding the direction of power flow at any given time of the day.

Fig. 2 shows the block diagram of proposed power corridor. The bipolar HVDC link consists of Converter_1W and Converter_2W on the west terminal side. Similarly, on the east side Converter_1E and Converter_2E are installed. Converter_1W is connected to Converter_1E. Converter_2W is connected to Converter_2E. When power is to be transferred from west to east terminal, the west converters are operated in rectification mode to convert AC to DC. The east converters are operated in inversion mode to convert back DC to AC. I1 and I2 depict the currents flowing through each circuit. Note that, the current flowing through earth is the difference between currents I1 and I2, which are in opposite directions. Usually, the system is operated in a balanced mode such that I1 is equal to I2, to ensure zero current flows through the earth terminals.

Whenever the power flow must be reversed, the operating modes of these converters are reversed by changing the firing angle of the power devices (thyristors or IGBTs).

All the converter inputs are connected to AC grid through 3-phase transformers to get the required DC voltage level. Higher the DC voltage, lower will be the line losses. Long distance bipolar links are usually designed to operate at around ±600kV or more.

On the AC side of each terminal, power is fed from solar-PV and wind farms. Also, there will be a load centre (city or industrial area) which consumes power.

HVDC vs grid batteries

As shown in Fig. 1, the power output of each plant provides output >80% for about 3.5 hours. Assuming that during this period, the power is being supplied to one load centre only, then the total duration for which >80% output is available is 5.5 hours. This means the system works as a battery for two hours. Assuming the average power in this duration is 2GW, the storage capacity works out to be 4GWh, which is a large capacity. Table 2 shows the performance comparison of grid battery with the HVDC corridor.

Implementation

Four examples of the proposed power corridor are given here. There may be many more possibilities based on the detailed study of the region where it must be implemented.

1. Two-terminal corridor

Fig. 2 is an example of a two-terminal corridor. Here, the total distance between eastern and western terminals is covered using a single HVDC link. It will provide maximum time difference between the two geographical locations.

2. Three-terminal corridor

Fig. 3 shows a three-terminal power corridor. In this scheme, one extra terminal is introduced in the middle of the corridor. The 3000km transmission line is now divided into HVDC lines of 1500km each. The central terminal should have potential to generate solar and wind energy. Also, a load centre should be present nearby to consume power.

In such a scheme, time difference will be about one hour from the central terminal on either side. If required, power can be transmitted directly from west to east terminals with a small increase in power losses. Such a corridor will provide more flexibility, and power availability will be more uniform throughout 24 hours.

3. Corridors with tappings

Fig. 4 shows a two-terminal corridor with a few taps introduced along the HVDC line. With advancements in HVDC technology, it is now possible to tap small amounts of power from the DC link. Up to 20% of power can be tapped. Beyond 20%, it becomes difficult to control the HVDC link. Such taps are useful for providing power to small load centres in rural areas located directly on the power corridor.

4. Augmentation of existing HVDC lines

We have several HVDC lines in use in our country. A detailed study of renewable power generation on these existing lines should be conducted. This study will provide clues as to which existing HVDC lines could be extended in the east-west direction. These are good candidates for expanding renewable power generation while utilising existing infrastructure.

Relevance

As the share of renewables in the grid increases, there is a need for adding battery storage at the grid level. Table 2 clarifies that the proposed power corridor can also act as a battery. It also offers additional advantages. Therefore, the proposed corridor also becomes relevant in the present scenario. When the link is idle, it can be used for transmitting power from conventional power plants. During emergencies, we can utilise this infrastructure.

As the share of renewables in the grid increases, there is a need to install grid batteries. These batteries help in stabilising the grid. An east-west power corridor with an HVDC link can extend PV power generation due to the time difference between east and west terminals. It acts as a battery and provides several advantages. It is necessary to evaluate this alternative for additional benefits. Hence, while planning newer HVDC links, the proposed power corridor should be considered as another possible solution. Furthermore, we can explore existing HVDC links for extension in the east-west direction to take advantage of the time difference in solar energy generation.

Dr Vijay Deshpande currently works for solar energy projects. He has worked as electronics hardware engineer in several companies