By Drew Robb

As coal and gas plants are taken offline to be replaced by wind and solar, grid stability and system strength can become serious challenges. This is due to the inherent inertia provided by rotating assets. To compensate, various approaches have evolved to provide the inertia, system support and stability the grid needs. These include capacitors, static VAR compensators, static compensators, a new batch of advanced electrical systems and transforming aging generators into synchronous condensing units.

Inertia and grid stability

Generators, motors and turbines provide inertia as they rotate at the same frequency as the electricity grid. Their presence acts as a buffer against power spikes and changes in frequency. During the evening peak, for example, frequency falls as people turn on air conditioning, heating, lighting and appliances. During the course of the day, frequency highs and lows must be balanced by grid operators to stay in the correct range (60 Hz for the U.S).

“In extreme cases, rapid changes in frequency can even take an entire neighborhood offline to maintain grid integrity,” said Morgan Hendry, President of SSS Clutch. “Failure to do so could damage equipment and if the situation worsened, lead to a regional blackout.”  

The potential for grid events has increased in recent years due to the growing presence of wind and solar. According to Industrial Info Resources (IIR), just five years ago the number of planned power generation projects to be built in the U.S. broke down to 57% renewables and 41% natural gas. This year, IIR reports that new-build power generation projects scheduled to begin construction in the United States between January 2024 and December 2028 will almost all be for renewable energy – 94%. That equates to 482 GW of new renewable generation by 2028.

Figure 1: Planned New-Build Generation, Renewables vs. Gas. Courtesy of IIR.

Unfortunately, the addition of wind and solar coupled with the removal of coal and gas plants increases the potential for grid stability and disruption. In wind, for example, frequency converters operate between wind turbines and the grid that prevent the kinetic energy of the wind blades rotating mass from providing inertia. There is also the factor of the many ups and downs in available wind and solar energy capacity. At some points in the day, there are massive amounts of capacity and at others, capacity falls away. This causes havoc for those dealing with grid stability who are tasked with maintaining voltage and frequency values across their networks. To do this, they must balance electricity production with consumption. Frequency rises if energy production is greater than the energy consumed and declines when more energy is consumed than produced. Such ups and downs make the grid susceptible to events such as sudden generation loss, load variation, ability to arrest frequency changes following a disturbance, grid frequency instability, lack of system strength or even cascading failures.

Peaking plants are on standby to take up the slack as a primary approach to maintaining grid stability. But the presence of rotating assets by itself also acts to slow any potential surges or plunges in grid frequency.

“Inertia is energy stored in a generator or motor which keeps it rotating,” said Steve Scrimshaw, Executive Director Siemens Energy UK & Ireland. “It helps slow the rate at which the grid frequency changes, as rapid changes can create instability in the system.” 

Another challenge to overcome is the location of wind and solar assets. Many U.S. wind and solar farms are far from load centers. Their output needs transmitted over long distances and that leads to system losses as well as reactive power issues. Reactive power can be regarded as the form of electricity that creates or is stored in the magnetic field surrounding a piece of equipment. It is measured in volt amperes reactive (VAR).

“Long transmission lines operating at heavy loads consume VARs,” said Hendry. “Failure to replace the lost reactive power leads to conductor heating, voltage failure, system instability or collapse, motor damage and electronic equipment failure.”

Improving the grid

There are a great many technologies and approaches in existence to address lack of inertia, grid instability, and reactive power while providing overall system support.

Capacitor Banks  

Drive by any electrical substation and you will see rows of capacitor banks (or shunt capacitors as they are sometimes known). They are inexpensive and reliable, hence their widespread deployment. But they aren’t enough. They eat up real estate, can only supply reactive power (not absorb it) and don’t do well on large load or voltage drops.

Static VAR Compensators (SVC) 

SVCs are basically switches that consist of a series of shunt capacitors and other electrical devices that improve voltage control capabilities compared to regular capacitors. Static VAR compensating devices can be placed close to power load to lower reactive current demand on the transmission system. They can absorb or supply reactive power. But they don’t respond rapidly to sudden changes in the grid and their reactive power output varies according to the square of the voltage. Hence, they struggle when addressing voltage instability or collapse. Some recent versions, though, are faster and more sophisticated as they can be customized to expected grid conditions and requirements. Hitachi Energy’s SVC control system, for example, can be utilized to control external shunt banks. GE’s SVCs, too, can be customized based on the utility’s technical requirements.

Static Compensators (STATCOM)

STATCOMs use power electronics and have a response time of a few microseconds compared to the slower mechanical solutions like capacitors. They are pricy compared to other options, but effective. American Superconductor’s Dynamic VAR (D-VAR) system scales from 2 MVAR and has overload capabilities of three times its rated capacity for up to three seconds. Hybrid systems combine SVC and STATCOM functions in one device. Hitachi Energy’s SVC Light Enhanced offers power quality and grid stabilization technologies as well as reactive power support.

Synchronous Condensers

A synchronous condenser is usually a large piece of spinning machinery composed of a generator and often paired with a flywheel to provide rotating inertia without generating any power. These machines spin at grid frequency to contribute to system stability by dampening frequency fluctuations and providing voltage stability through reactive power.

“Synchronous condenser is the name given to a synchronous machine that is connected into an electrical network to help in maintaining the system voltage,” said Dr. James F. Manwell, Emeritus Professor of Mechanical and Industrial Engineering at the University of Massachusetts, Amherst. “The synchronous machine is essentially a motor to which no load is connected.” 

Vendors like GE Vernova, Siemens Energy and Hitachi Energy provide different approaches to synchronous condensing. Siemens Energy’s solution is comprised of a horizontal synchronous generator connected to the high-voltage transmission network via a step-up transformer. It is started up and stopped with a frequency-controlled electric motor (pony motor) or a starting frequency converter. When the generator reaches synchronous speed, it provides reactive power to the transmission network as well as inertia and active power injection or absorption during sudden load unbalance events. GE’s synchronous condenser/flywheel combo is air cooled and rated up to 300Mvar+.

Siemens Energy provided a synchronous condenser to help stabilize the UK grid. Courtesy of Siemens Energy.

Using Existing Generators

With so many steam and gas turbines being decommissioned, a popular approach to grid stability is to maximize the investment in these rotating assets by fitting them to operate as synchronous condensers. Instead of discarding these machines, a synchronous self-shifting (SSS) clutch can be added to disengage the generator from the turbine. The turbine brings the generator up to speed so it synchronizes with the grid, at which point the turbine disconnects from the generator and shuts down. The generator then uses grid power to keep spinning, constantly providing leading or lagging VARs as well as other forms of grid support and the needed inertia. When active or real power is needed, the SSS Clutch automatically reengages for electric power generation. This feature is useful in renewable focused grids where there may be a sudden need for peaking power.

“For coal plants being closed down, the steam turbine generator can be easily converted to a synchronous condenser by removing the turbine and adding an acceleration drive with an SSS Clutch,” said Hendry.

New gas-fired power plants being built can also be configured to operate as a synchronous condenser. Hendry listed 45 recent clutch orders intended for GE LM6000 PF+ Sprint models. In those cases, the clutch is built into a load gear as the unit operates at a higher speed than 3,600 rpm, which is needed for a 60 Hz. As result, a gear is needed to create synchronization with the grid.

By far the biggest recipient of these clutched LM6000 PF+ units is the Tennessee Valley Authority (TVA). It has received 10 SSS Clutches so far and another 20 are on order. Reason: The TVA is in the midst of rolling out 1 GW of wind turbines and solar PV in Tennessee and decommissioning coal and other rotating assets. Its new order of LM 6000s are there for peaking power to support. By enabling them to run as synchronous condensers, the TVA is ensuring it has enough inertia, system stability and reactive power support. Far from being a novel arrangement, the SSS Clutch being connected to the load gear is a well-established practice.

“The mounting arrangement in the load gear is the same as nearly 300 Frame 5 and 6s gensets GE has done in the past for synchronous condensing,” said Hendry.


About the Author: Drew Robb has been working as a full-time freelance writer in engineering and technology for the last 25 years. For more information, contact drew@robbeditorial.com.

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