By Kathryn Porter, Watt-Logic

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.

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.

This post was co-authored by James Porter.

Original article   l   KeyFacts Energy Industry Directory: Watt-Logic