Recent posts have explored some of the issues the US power grids are facing as the premature closures of fossil fuel and nuclear power stations are creating problems for security of supply. Something discussed less often in this context is the impact on grid stability – while the mainstream press eagerly covers news of potential blackouts due to energy shortages which are easy to understand, they are less quick to report on the effects of renewable generation on grid stability. These adverse effects were clearly seen in Texas last week when emergency measures were needed to correct a significant grid frequency deviation, and are leading to increased interest in the use of synchronous condensers.
Synchronous condensers are machines which provide the same benefits to power grids as conventional generation, without actually providing electricity. In other words, they provide inertia, reactive power and short circuit level resources current inverter-based technologies fail to deliver. While that may change with the introduction of grid forming power electronics, those technologies are still immature.
Over the past decade, the United States has converted several decommissioned power stations into synchronous condensers. For example, in 2014 FirstEnergy converted mothballed coal-fired units at Eastlake, near Cleveland, Ohio into five synchronous condensers. In response to the sudden closure of the San Onofre nuclear power station in California, two units at the recently closed Huntington Beach power station were converted to synchronous condensers, although they were later taken out of service in 2017 after new synchronous condensers were built at the Santiago and San Luis Rey substations. Two synchronous condensers were installed in the Texas Panhandle on the Oncor substation network to stabilise the effects of wind power in the region.
What are synchronous condensers?
A synchronous condenser is essentially a motor with no connected load or a generator and without a prime mover, which produce or absorb reactive power, provide additional short-circuit strength and deliver mechanical inertia. In high-inertia synchronous condensers, additional inertia is often provided through the addition of a flywheel. The typical reactive power rating for synchronous condensers is between 20 and 200 MVAr, but tailored solutions up to 350 MVAr can be provided.
Synchronous condensers are a long standing well-understood technology which can remain connected to the power grid and provide reliable operation even under extreme low voltage contingencies. They can also provide dynamic fast frequency response services by using modern excitation and control systems and real short short-circuit strength to the grid. They are not a source of harmonics and can even absorb harmonic currents. They do however have some drawbacks, primarily having higher levels of losses, mechanical wear and a slower response time compared to batteries or power electronics.
Synchronous condensers can be built directly, but they can also be developed by re-purposing fossil fuel generators, if the configuation is suitable. Where new synchronous machines are being built, it makes sense to include a clutch that would allow the plant to switch between generation and sychronous condesing modes. The clutch is inserted between the turbine and the generator – disengaging the prime mover and the generator when reactive power is needed, and re-engaging it for power generation when real power is needed. This addition to the design comes with a modest additional cost, which is often small in the context of the wider project cost. It then provides the plant with the flexibility to offer grid serves seperately to the provision of energy.
Synchronous condenser use in Texas set to grow
Inverter-based resources in west Texas have experienced rapid growth, with the total capacity projected to exceed 42 GW by the end of 2025. The prevalence of these resources, primarily wind and solar, together with the absence of conventional synchronous generation can weaken electricity system and increase the likelihood of instability. The west Texas region experienced notable disturbances in 2021 and 2022, specifically the Odessa events, which unexpectedly led to a substantial reduction in power output from renewable generation triggered by the widespread propagation of low voltages during single-line-to-ground fault conditions. Some of the renewable generation impacted by these events as located some distance away from the fault location.
In the 2021 Odessa event, a single-line-to-ground (Phase A) fault occurred on a generator step-up transformer at a CCGT near Odessa, Texas. The fault was caused by a failed surge arrester at the combustion turbine during startup for testing. The circuit breaker for turbine 1 operated and cleared the fault within three cycles and the unit 2 experienced a partial trip followed by a run back for a total loss of 192 MW. The fault caused voltages in the area to drop to 0.72 pu at the 345 kV connecting station for the generation facility, 0.84 pu around Fort Stockton at a 138 kV station, and as low as 0.54 pu at a 69 kV bus near Alpine, Texas. Voltage in the area recovered to near pre-disturbance levels very quickly (within a couple electrical cycles) after the fault cleared.
In addition to the generation loss at the CCGT, several solar PV and wind plants also exhibited active power reductions caused by the fault event. None of the affected inverter-based resources were tripped consequentially by the fault itself. Rather all reductions were due to inverter-level or feeder-level tripping or control system behaviour within the resources. Active power reductions were: CCGT – 192 MW, solar – 1,112 MW and wind – 36 MW. A significant factor in these losses was a failure by many PV operators to follow NERC reliability guidance and a failure of grid operators to include performance requirements in connection agreements.
In the 2022 Odessa event, a surge arrestor failed at a synchronous generation facility in Odessa, causing a B-phase-to-ground fault on the 345 kV system. The fault cleared in three cycles, disconnecting part of the plant that was carrying 333 MW. Other units in the plant unexpectedly tripped for an additional immediate loss of 202 MW. A separate synchronous generation facility in South Texas over 450 miles away lost an additional 309 MW. In total, 844 MW of synchronous generation tripped at the time of the disturbance.
In addition, 1,711 MW of inverter-based resources from many different facilities also unexpectedly reduced output due to protection and controls at each site. Hence, the normally-cleared single-line-to-ground fault resulted in a total loss of 2,555 MW of generation, and system frequency dropped to 59.7 Hz. The total responsive reserve service available at the time of the disturbance was 2,442 MW. Total responsive reserve service deployed was 2,343 MW with 1,116 MW from load resources and 1,227 MW from generation. In all, 844 MW of synchronous generation and 1,711 MW of solar generation was lost in the incident.
The Electric Reliability Council of Texas (“ERCOT”) has performed a study to strengthen the system in west Texas and to address these operational challenges, and has concluded that the installation of new synchronous condensers at the Cottonwood, Bearkat, Tonkawa, Long Draw, Reiter, and Bakersfield 345-kV substations should be progressed to bolster the reliability of the west Texas system. ERCOT recommends the following locations and engineering specifications for the new synchronous condensers:
- 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 (MW-s) or above at each location, incorporating synchronous condenser with flywheel
- Effective damping control to meet the ERCOT damping criteria in the Planning Guide
The modelling showed that new synchronous condensers at these substations delivered the best outcomes across the region. These locations are strategically spaced, with a significant number of major transmission connections, ensuring optimal distribution of reactive power support across the region. The analysis indicated that both these improvements and a continued focus on improving the capability and performance of inverter-based resources are needed to maintain the reliable operation of the ERCOT system, and that additional improvements will be required to support the continued growth of such resources in the ERCOT system.
There are plans for construction of new gas peaking generators in Texas to provide additional energy as well as grid support. These new synchronous machines would be more efficient if they included the capability to operate as synchronous condensors as well as generators, something which is easier to achieve if specified as part as the initial design, rather than trying to retrofit the necessary clutch later. Despite plans to close larger fossil fuel generators across the US, demand for peaking plant is increasing – installing them without the capability to function as synchronous condensers would be a missed opportunity.
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The fluctuations and instabilities observed on US power grids in recent years are increasing as conventional generation is replaced with intermittent renewables. Synchronous condensers are a straightforward way of mitigating these effects, and can be delivered through re-purposing retiring thermal generation if the physical layout of the plant is suitable for conversion. Grids with large volumes of inverter-based resources such as the west Texas region are likely to require the addition of synchronous condensers to maintain grid stability if the transition to a de-carbonised grid is to continue without compromising reliability. Where new peaking plant is being installed, including the capability to work as a synchronous condenser will allow these new facilities to deliver the maximum benefit to the grid.
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This post was co-authored by James Porter.
Another very interesting article, thanks Kathryn, but this time a little over my head technically. Do synchronous condensers do the job of energy storage? If so, do we know how efficient they are? People talk very glibly about storage but tend to gloss over the conversion losses, particularly with hydrogen. I guess these condensers are not a substitute for other forms of storage.
Hi Steve,
The way to look at this is to think about how electricity is delivered in a conventional grid. Alternating current is produced when one magnet rotates inside the magnetic field of another magnet. The result is current (and voltage) that varies over time with a sine wave pattern. In power grids with a 50 Hz frequncy, there are 50 cycles per second over which voltage and current complete a single “wave”. This is produced by turbines rotating at 3,000 RPM (except for nuclear which is at half that speed).
Lots of electrical equipment connected to the grid required the frequency with which voltage and current change to be constant within a narrow range, otherwise protection mechanisms activate to disconnect teh equipment to prevent it from being damaged. This means grid operators need to maintain supply and demand in close balance because otherwise this freqency is disrupted.
The benefit of large rotating machines is that they not only create that voltage and current shape but because they are big and heavy, they resist attempts to change their speed of rotation, ie they have what’s called “inertia”. This helps to keep grid frequency stable and prevents damage and disconnection by conencted equipment (which would ultimately lead to blackouts).
However, wind and solar produce direct rather than alternating current. Their electricity is injected into the grid using power electronics which observe the waveform: if it is behaving as expected they inject their electricity into the grid, if not they either fail to connect if they weren’t already running, or disconnect if they were. They do not contribute to maintaining the waveform. (It is hoped that new power electronics will be able to do this, but at the moment they don’t)
Not only that, these renewables displace conventional rotating generators from the grid, so there are fewer machines creating the waveform and fewer machines resisting changes to it. This makes the grid less stable, and increases the risk that frequency moves outside acceptable operational tolerances, potentially leading to blackouts. Typically it is a voltage disturbance that causes a blackout – which is generally caused by a sudden large difference in supply and demand eg because a large generating unit unexpectedly trips or because a fault is not resolved quickly enough.
Synchronous condensers help because they are big heavy rotating machines, but they do not generate electricity (although some can). They provide extra inertia to the grid, helping to stabilise grid frequency. They don’t do the job of energy storage although some storage such as hydro also involves synchronous rotating machines, and the injection of power from batteries can help to mitigate the effects of frequency deviations. But they don’t help to prevent those deviations passively the way synchronous condensers do – synchronous condensers just sit there, rotating in synchronicity with the grid, providing heft for want of a better word.
Hope that helps!
The UK charged for VAr at punishing rates – but not for domestic consumption – so capacitor banks were commonly used thus virtually all industrial power would have a power factor very near to 1.0
I’ve been shocked to see that some LED lights are being sold with power factors as low as 0.5. At least they don’t draw much power I suppose.
Apparently power consumption in the UK might double by 2050 and all of that will be inductive load from consumer consumption for heat pumps, EVs etc which will require huge power correction. As it was explained to me, when the reactive load gets too high, the (inertia capable) generators start going backwards.
Timely warning, Ms Watt!
The obvious solution is to force the manufacturers of such equipment to have a power factor of 1 at all times. It could well be that serious.
I noted that National Grid recently commented that they had spent a lot in the Balancing Mechanism on handling frequency excursions in Scotland, where generation is often dominated by wind power. Having one part of the grid operating out of sync with another potentially produces enormous voltage and current excursions due to interference effects between the two frequencies which can lead to power transmission effectively being blocked and thermal overload in the exporting area, since the energy is not flowing away it must heat the power lines. So they have been forced to curtail wind generation, and pay for replacement generation further south. Of course, there are major synchronous condenser projects such as Cruachan, and Blackhillock/Keith and others, but it is all running rather behind the scale of the problems.
I have long noted that the pumped storage on El Hierro, which was meant to be a 100% renewables island has long been repurposed to provide grid stabilisation when it is windy: Water is pumped up one penstock to the upper reservoir to help absorb excess power, while being left to flow back down again in the other at the same time to provide inertia without adding generation. The effect is similar to pushing on a door at the same time as you pull on the handle to close it gently – perhaps against a howling gale outside. Although this chart is mainly an illustration of extended Dunkelflaute on El Hierro last December you can see the pumping (negative light blue) far outweighs pumped storage generation because the water has already flowed back downhill again. The regime is somewhat forced by the limited size of the lower reservoir in particular – it must have water to pump to absorb wind surpluses.
https://i0.wp.com/wattsupwiththat.com/wp-content/uploads/2023/06/El-Hierro-Dec-22-1687452220.2666.png
Very nicely explained, Kathryn. Thanks for taking the trouble.
Now that was definitely a worthwhile article, well explained and presenting a cost effective solution to current problems.
If we are counting costs of renewables, and we have to look at the whole system costs, how expensive are these sychronous condensers, and if you add their separate cost into the cost of conversion to renewables, do they make renewables uneconomic, or is it that actually if they were used as converted/hybridized CCGT units, the cost would be insignificant because the CCGT units are needed for back-up and at all other times they could be acting as grid stabilization services?
Would hybridized CCGT actually be a very cost-effective solution for grid stabilization, and only mean that the changes to capital investment and OPEX needed compared to now would be minor?
Should it be a technical demand by the system operator that all peaking/balancing and CCGT also have stabilisation capability? Would that then reduce the costs associated with peaking/balancing.
If we need grid stabilization in certain locations….more distributed than now, would it be better to relocate the CCGT to those same locations as hybridized CCGT/synch conds?
There are obvious synergies that can be introduced into the system, if they are adding additional capacity/functionality as separate entities now, with separate CAPEX and OPEX this would be obvious fertile ground for industry rationalisation in the future.
The only benefit of any short-term inefficiency is it makes the GDP and employment look good for a while, but it will add to the cost of electricity bills, due to the inherent inefficiency.
There are different ways to manage this process and technology implementation, and a single overview would be useful. Whilst it may be useful for the National Grid (or whoever is responsible for putting out the requests for these services, and running the auctions/planning) where it is all done piecemeal because of the target dates and the demanded break-neck speed of the changes in infrastructure, with each new service as an additional spur off the grid, hopefully these newly introduced inefficiencies should come out of the system over the longer-term, if the system is being managed to minimise the cost of electricity.
Operational synergies are essential to get the best out of the system, i.e. the greatest efficiency, and the lowest cost of electricity
Very good article how much noise is there from these synchronous condenser we have one being built soon in south wales countryside