Let This Sync In

ERCOT recently published a report that identified the installation of synchronous condensers at six locations on the Texas power system to address operational and reliability challenges associated with the large volumes of wind and solar generation on their system. This is an excellent example of the evolving nature of power system planning and operations so I think it's worth some discussion.

Degrading Power System Characteristics

ERCOT has seen a significant increase in inverter-based resources (IBRs, wind and solar generation) connecting to the Texas power system in recent years, and they have reached a point where there are regions of the system that are dominated by IBRs with little to no synchronous generators. In these regions, the strength of the system has been weakened with decreased inertial response and reactive power support which is creating operational challenges that make it difficult to maintain reliable operations and manage system disturbances.

Texas is not alone in its challenges with large penetrations of IBRs. There have been 13 significant disturbance events associated with IBRs on the Western Interconnection (WECC) and ERCOT combined since 2016, the largest of which was the Odessa Disturbance in 2022. The graph below shows 12 of the disturbances and doesn't include the Southwest Utah Disturbance that happened in April 2023.

These large disturbances demonstrate the growing reliability risks associated with increasing volumes of IBRs and decreasing volumes of synchronous generators on power systems. FERC's recent Order No. 901 directing NERC to develop specific reliability standards for IBRs recognizes this current and growing threat to reliability.

Given the current challenges and the large number of IBRs planning to connect to the Texas power system in the near term, ERCOT has identified the installation of a number of synchronous condensers at specific locations on its system as a near-term action to help maintain reliable operations.

What's a Synchronous Condenser?

A synchronous condenser is essentially a conventional generator that is not coupled to a turbine to provide energy, but simply synchronized to the power system to freely spin at system frequency (60 Hz, 3600 RPM) to provide inertia, reactive power, and short circuit support. Synchronous condensers can also be fitted with flywheels to provide additional inertia beyond the inherent inertia provided by their rotating masses.

Synchronous condensers (aka synchronous compensators, synchronous capacitors, or syncs) are not new, and have been used on power systems since the 1920s to provide voltage control and inertia and many are in operation today. When I was a maintenance engineer at Manitoba Hydro in the early 2000s, I got to balance synchronous condensers at HVDC converter stations when they went back into service after maintenance. However, because power systems have historically been dominated by synchronous generators that provide inertia and voltage support, the use of additional synchronous condensers to provide reactive power support gradually gave way to more cost-effective technologies such as static VAR compensation or STATCOMs, which are based on power electronics that are faster acting and cheaper to operate and maintain than big spinning masses. As with many technologies, "analog" syncs gave way to "digital" STATCOMs.

But with the proliferation of "digital" IBRs (wind, solar) driven by the energy transition, the good old "analog" physical operating characteristics of synchronous condensers can now help compensate for the decreasing number of synchronous generators on power systems to help system operators maintain reliable operations.

ERCOT describes the characteristics of synchronous condensers in their report as follows:

• Synchronous condensers’ response to system disturbances is governed by the physical characteristics of the devices, which makes them more reliable and predictable than other device technologies that have a response based more on their control system programming. Inverter-based technology like grid-following inverters and STATCOMs are both based on power electronics control, which can be more susceptible to system conditions and unpredictable.
• Synchronous condensers are rotating machines, providing a strong system inertia which is important to keep the system frequency stable.
• Synchronous condensers provide dynamic reactive power support on the system by absorbing or generating reactive power as needed.
• Synchronous condensers improve system strength and increase fault current which is beneficial for the proper functioning of IBRs.
• Synchronous condensers are a mature technology with a long history of successful use.
• Synchronous condensers are being considered and implemented globally by the utility and system operators with a high penetration of IBRs. In addition, various countries in Europe (e.g., Germany, Latvia, Lithuania, UK, Ireland, Estonia, and Italy) are considering and have already implemented synchronous condensers augmented with flywheels in the last several years to improve the reliability of their systems.
• Flywheels, mechanically linked with synchronous condensers via couplings, provide additional inertia and are beneficial for frequency support. This addition is a valuable measure to prepare for uncertain system conditions that may arise in ERCOT.

Synchronous condensers are not new to the Texas system. In 2018, two syncs were added in the panhandle region to provide voltage support and increase system strength.

ERCOT Recommendations

The ERCOT study makes the following synchronous condenser recommendations:

• New synchronous condenser at six locations: Cottonwood, Bearkat, Tonkawa, Long Draw, Reiter, and Bakersfield 345-kV substations
• Approximately 350 MVAr capacity at each location
• Around 3,600 Ampere of three-phase fault current contribution to the 345-kV point of interconnection
• A combined total inertia of 2,000 MW-seconds or above at each location, incorporating synchronous condenser with flywheel
• Effective damping control to meet the ERCOT damping criteria in the Planning Guide Section 4.1.1.6

ERCOT states that adding new syncs at these specific locations on the Texas power system will "effectively bolster the reliability of the West Texas system, make the system more resilient to unexpected events, and address the challenges that may arise in real-time operations."

Concluding Thoughts

The increasing number of IBRs connecting to power systems around the world as part of the energy transition is creating new reliability and operability challenges for power system operators who have historically relied on synchronous generators to help maintain system reliability. However, the revival of good old synchronous condenser technology can help, and we are seeing new syncs being installed on power systems in Texas, Ireland, Maine, and others. This is just one example of the kinds of adaptations system operators are making as our power systems continue to evolve. In the long term, power the power system evolution will require a combination of existing technologies, such as syncs, and new technologies we haven't even thought of yet. Suffice it to say, power system planners and operators are certainly living in interesting times :-)