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Design considerations for utility solar farms in Zimbabwe

   

The picture shows a simplified utility solar farm that consists of a single grid-tied solar PV (photovoltaic) inverter connected to a transformer that is connected to the national electric grid. Each solar inverter is connected to thousands of solar PV panels.

According to recent media reports, some progress has been made on the long-awaited Gwanda solar project as evidenced by the deal that was signed by Intratrek Zimbabwe, Chint Power Systems and Zimbabwe Power Company (Sibanda 2015). Adding another 100 MW to the grid sounds great in theory until you look in the details and the controversy surrounding the project (Share 2016). The success of the project depends on the diligence and technical ability of stakeholders during the design, construction and operation phases. Solar engineers must consider factors such as the impact of load-shedding, intermittency of solar energy, cost, existing electrical noise on the grid, local climate, state of existing electric grid transmission and distribution infrastructure, risk of vandalism, risk of theft, long-term plant operation and maintenance strategy and bankability of the main contractor. I shall restrict my attention to the technical details of two factors: load-shedding and intermittency of solar energy.

Load-shedding

Load-shedding is the intentional shutdown of some areas of the national electric grid when there is a power shortage. As counter-intuitive as it may sound, load-shedding may limit the performance of the proposed Gwanda solar project. The reasons are found in the technical requirements of large utility solar farms, also known as solar photovoltaic (PV) grid-connected power plants. Utility solar farms, like the proposed Gwanda solar project, convert sun’s energy into electricity (direct current) using thousands of solar PV panels; change the electricity into a grid-friendly form (alternating current) using hundreds of special solar inverters also known as grid-tied solar PV inverters; and then transmit the electricity to the national electric grid to meet demand. The proposed Gwanda solar power plant must be connected to the grid because consumers are on the grid and it is expensive to store such large amount of electricity in batteries.

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Technical challenges arise when the grid, which the solar PV power plant is connected to, is turned OFF during load-shedding. Most grid-tied solar inverters such as those likely to be used in the Gwanda solar project (Chint Power Systems’ CPS SCA500/630kW) are required to shut down when the grid shuts down (anti-islanding feature) as per IEEE 1547, IEC 62109 or IEC 61727 standards. Why shut down the solar inverters when the power is most needed? Well, two reasons: (1) to protect utility workers who may be doing repair work on the same electric grid as the inverters and (2) to prevent the solar grid-tied inverters from going out of sync with the electric grid’s voltage and frequency. The first of the aforementioned reasons comes natural to most people who have changed a light bulb or installed a new electrical socket in a house—always turn the main switch OFF before any repair work! If the grid is shut down, the solar inverter is shutdown as well and so there is no power to transmit to the national grid.

In the context of the current load-shedding schedule of 4AM to 10PM, the proposed Gwanda solar power plant will not be transmitting power to the grid from 4AM to 10PM (Mawonde 2015). Therefore, the proposed Gwanda solar power plant may not serve its purpose unless drastic measures are put in place to mitigate effects of load-shedding.

From my experience and training in the solar PV industry, I am not aware of plans to bypass anti-islanding features in order to address load-shedding schedules that last several hours as experienced in Zimbabwe. However, solutions exist for temporary grid shutdowns and instabilities that last only a few seconds; solar grid-tied inverters are equipped with low-voltage ride through (LVRT), zero-voltage ride through (ZVRT) and/or frequency ride through (FRT) technologies to prevent the solar power plant from shutting down due to minor disturbances on the grid (Bank, et al. 2013). Chint Power Systems’ CPS SCA500/630kW inverter has the zero-voltage ride through capability meaning that it can continue to operate when the grid voltage is low for a few seconds.

The best solution is to connect the solar power plant to a transmission line that is ON and has stable voltage and frequency all the time. How many transmission lines meet this criterion in the Gwanda area? Whatever the answer may be, solar engineers should be prepared to deal with occasional disturbances even in the most stable grid.

Intermittency of solar energy

The saying “as unpredictable as the weather” is true with electricity from ordinary solar PV power plants. If thick clouds form, then immediately the solar power plant produces very little or no electricity and thousands of households are left in the dark. The same scenario occurs when a layer of dust settles on the sunny side of solar panels. What about at night? The public cannot surely depend on solar energy at night unless the solar power plant is equipped with expensive back-up batteries.

The challenge is magnified if the solar power plant supports a big portion of the total power production on a given section of the electric grid. As an analogy, a person who earns $200 is more devastated than a person who earns $3000 if they both lose $100 to a thief. In the same way, on cloudy and dusty days or at night, loss of solar power may lead to unexpected and devastating power shortages to the users of the Gwanda solar power plant.

In response to sudden loss of solar power, the grid operator (Zimbabwe Power Company in this case) turns ON big diesel generators (or any other form of peaking generators) to make-up for the ‘lost’ solar PV electricity and in order to keep consumers satisfied. The success of this solution depends on the availability of diesel generators of sufficient capacity, fast communication technology, and reliable weather prediction tools. In summary, advanced and often expensive technology, much of it is already in use in modern electric grids, is needed to deal with the intermittence of solar energy.

In conclusion, the success of the proposed Gwanda solar project depends on how much stakeholders mitigate the effects of load-shedding, intermittence of solar energy and a host of other technical, legal and political factors. Fortunately, we, as a country, can avoid common pitfalls by learning practical lessons on the integration of renewable energy from places that have high percentage of renewable energy power plants such as Hawaii, Arizona, California, Denmark and Germany (Bank, et al. 2013) (Hawaii Electric Companies 2014) (Public Utilities Commision: State of Hawaii 2013).

References

  • Bank, J, B Mather, J Keller, and M Coddington. High Penetration Photovoltaic Case Study Report. Technical, U.S. Department of Energy, NREL, Golden, CO: NREL, 2013, 15-19.
  • Hawaii Electric Companies. Transient Over-Voltage Mitigation: Explanation and Mitigation Options for Inverter-Based Distributed Generation Projects ≤ 10kW. February 24, 2014. https://www.hawaiianelectric.com/vcmcontent/StaticFiles/pdf/TOV-Mitigation-Measures-Public-Brief-Feb-24-2014(Final).pdf (accessed October 6, 2014).
  • Public Utilities Commision: State of Hawaii. “Annual Report: Fiscal Year 202 – 13.” Annual Report, 2013, i,37.
  • ZPC. Generation Statistics. Zimbabwe Power Company. October 25, 2015. http://www.zpc.co.zw/ (accessed October 25, 2015).

This guest article was written by Jonah Kadoko. He is a mechanical engineer with a strong passion for power generation systems. Currently, he is pursuing graduate studies in mechanical engineering at Tufts University. He has experience in the design of commercial scale solar PV grid-tied inverters and solar power plants from his training in solar engineering and previous role as a mechanical design engineer at Solectria Renewables, MA USA.


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