Back when we had a fair number of large coal-fired power generation stations near city centers, it wasn’t that difficult to balance the grid. As I understand it, the torque in those big turbine generator sets had so much spinning mass that the grid was somewhat impervious to most disruptions. But now we are closing down numerous bulk power generation plants, partly because utilities have sold off generation and became T&D-only entities; partly because of environmental restrictions; and partly because aging nuclear and fossil-fueled plants are not as competitive in today’s energy market, which now includes alternative energy sources (natural gas, wind and solar). Therefore, electric utilities are coming up with creative ways to continue to provide stability to the transmission grid.
I had the great opportunity to join several of my peers to visit both an operating static VAR compensator (SVC) and an operating synchronous condenser (SC), both located just outside of Cleveland, Ohio, U.S.
The SVC was located at FirstEnergy’s Lakeshore facility, which is located at the former site of the company’s Lakeshore coal-fired generation plant. Eric John with ABB provided technical detail on the SVC in operation. The reason for building out the Lakeshore SVC is fairly straightforward, FirstEnergy wanted to accomplish three things:
• Regulate and control the 138-kV system voltage to the required set point under normal steady-state and emergency conditions
• Provide dynamic, fast-response reactive power following system-contingency conditions
• Enhance first swing stability by maintaining system voltages during large disturbances.
A SVC regulates the system voltage by controlling the amount of reactive power absorbed from or injected into the power system. It generates reactive power when the system voltage is low or loads are inductive, and absorbs reactive power when the system voltage is high or loads are capacitive.
The FirstEnergy Lakeshore SVC operates at 21 kV and provides -150 MVAR to +260 MVAR reactor power to the transmission system. Thyristor-controlled capacitors and reactors make up the bulk of the SVC feeding into three single-phase step-up transformers. Harmonic filter banks clean up the waveforms to keep from pushing harmonic distortions onto the 138-kV transmission system.
The Lakeshore SVC system includes power transformers, thyristor valves and capacitors from ABB; reactors from Coil Innovations; and disconnect switches from Southern States. The SVC went live in June 2015 and has been delivering the desired voltage support and stability to the grid.
As much as I enjoyed learning about the FirstEnergy SVC, seeing an SC up and running was déjà vu. Power companies including FirstEnergy can essentially leverage the generator portion of an existing power station to provide voltage regulation and stability by providing continuously adjustable reactive power and also maintain existing short-circuit strength.
The SC system we toured was converted by GE for FirstEnergy at its former Eastlake generating station. Richard Bodo with GE provided us with a detailed understanding of the value of SCs. Bodo stated that there is a lot of interest in SCs today to strengthen the grid, especially with the need to support renewable generation. The benefits of converting retired generation to SCs provide many benefits to the transmission system:
• Maintains system short-circuit MVA
• Quickly responds to power system fluctuations
• Offers ride-through capability
• Has short- and mid-term overload capability while providing system reliability, service life and maintainability.
The Eastlake facility provides between +1200 MVAR and -600 MVAR of reactive support from the five units that were converted and placed in service between 2013 and 2016. The SCs have performed well since going into service and have successfully provided needed dynamic reactive support to the area. This was a particularly ideal location to consider implementation of this technology because of the condition and location of the units, and the number of units being retired.
An SC is made up of several important components, in addition to the retired generator portion of the unit. Of course the turbine is disconnected from the generator and the exciter is replaced with a modified turning gear assembly. Then a computerized controller and a variable frequency drive are used to manage the operation of the unit. This simplified configuration is dependable and will provide many years of reliable unit operation from the repurposed fossil-fuel-powered equipment.
In addition to the SVC and the SC installations I visited, our industry also installs static synchronous compensators (STATCOMs) to provide stability to the grid.
In an interesting twist, Dominion Energy is presently working with Siemens to develop a mobile STATCOM. This will enable Dominion to stabilize the grid when affected by disturbances and faults by providing fast and controlled reactive power. Because this STATCOM is mobile, it can be moved to anywhere grid support is needed, such as responding to the impact of renewable generation sources or responding to unexpected outages from extreme weather or unplanned events.
As the industry changes, it’s good to know we have the technology, the wherewithal and the creativity to help maintain the stability of the grid as utilities including FirstEnergy and Dominion so aptly demonstrate. ♦
