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Solar Inverter: Regulating Voltage for Transmission Lines

published: 2011-10-20 20:44

One major challenge for utility-scale solar developments, across the global community, concerns access to high-voltage transmission lines. Transmission lines move power from the generation plant to the area of power demand. The lack of available transmission capacity presents a major impediment to development of large-scale solar projects.

The technological solution to fixing this problem must help improve the transient stability -- the power system's capacity to overcome major disruption, and preserve synchronism. It must also enhance the power distribution abilities for the utility company management.

A University of Western Ontario engineering professor, Dr. Rajiv Varmahas developed a solution, which relieves transmission congestion often experienced by wind and other renewable energy power plants. The concept behind the device is to take a photovoltaic farm to function as a static synchronous compensator (STATCOM). STATCOM consists of a regulating mechanism commonly used on alternating current (AC) transmission system around the globe.

If the technology proves successful, it could mean another source of revenue stream for solar developers by putting tens of millions of dollar worth of capital equipment to work, serving another function during what have traditionally been periods of idleness. Dr. Varma claims the simulation of the modified inverter on a 100-megawatt solar farm boosted the nighttime transmission capacity of the system to 150 MW. He recently applied for two U.S. patents for his groundbreaking invention.

The Traditional Method

The most direct solution for distributing increased electricity capacity involves the construction of new transmissions lines. However, building new lines entail a complex, long, and expensive process.

Advances in technology have allowed for increased transmission over the same line. It requires a flexible alternating current transmission system (FACTS), but developers do not install these devices on any significant level anywhere in the world.

FACTS devices can add millions to project, which is still well below the cost to build new transmission lines. FACTS devices rely on an inverter, which has a controller, motherboard-size circuit board, and computer software.

Often, wind power plants generate more electricity than required, to meet the current level of demand. This scenario usually occurs at night when demand decreases. Wind gusts increase the system's voltage, which causes variations on the line that connect the power plant to the grid and other lines as well.

Dr. Varma stated in an interview, “Our lines do not have enough capacity to add this new generation.” The pervasiveness of the problem motivates many wind-turbine developers to include the cost of FACTS in their projects. This makes the development more attractive when attempting to gain access to the utility grid.

The New Technology

The  new solution requires paired hardware and software programs that track the photovoltaic power station's inverter, which regulates power for thousands of dollar compared to current solutions that run into millions. The device does two things: regulate voltage on the line and improve transmission capacity.

The inverter technology required to retrofit a solar farm provides a simpler, less expensive solution. By itself, an individual solar farm cannot manage fluctuations, which routinely occur on a transmission line. However, taking the cumulative spare capacity on a line, which includes power not generated at the installation, makes the solution feasible.

Dr. Rajiv Varma's, the inverter on solar farm converts the DC solar power for output to grid as AC power. The new technology can also condition other power on the line that comes from another source. The excess power can come from an entirely separate transmission line. The only requirement is that the power plants connect to the same sub station plant.

Dr. Varma's research team works with Hydro One, Ontario's transmission system operator and   thin film manufacturer First Solar. The team performed sophisticated computer modeling to prove they could configure solar PV inverters to function as STATCOMS. The system includes proprietary software, which upgrades the physical hardware.

The next phase of the project focuses on the development of ten prototypes. Varma's team plans to conduct laboratory testing before undertaking any field testing. The team has installed modified solar inverters on the distribution networks of two of its partners - Bluewater Power and London Hydro. The group formed a new firm -- SMG Night Solar to bring the product to market.


The technology requires a new control system for the solar inverter and cost about $50,000. Wind plant operators, and utility companies clearly win with this technology. Varma says solar inverters installed in 4.5 megawatts solar plant could expand the capacity of the distribution line to allow an addition seven megawatts of wind- turbine generated electricity from an otherwise congested  network.

Instead of ramping down production during the evening hours, the plant remains in operation. Manufacturers can improve the functionality of inverters on the production line or operators can have inverters for existing power plants retrofitted with the technology.

Dr. Varna and his partners plan to roll out the modified inverter with distribution systems, which allow solar operators to control the voltage. The team plans to apply the system directly to transmission lines. However, transmission line operators will have to change current codes to allow for the required testing on transmission feeders. A 20 MW solar power plant can supply the voltage regulation service for a 100 MW wind farm.

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