Synchrous condenser cutaway. Courtesy of Siemens

Inertia Means Stability

Jan. 15, 2021
The digital grid has a lot of renewable generation and more is coming every day.

Anyone who has ever stood on the turbine deck of a power plant with a large steam power generator running at full load, knows, firsthand, the meaning of kinetic energy (energy due to its motion). It’s an amazing feeling, everything around the generator vibrates, the floor, the walls, the people, even the air. There is something mesmerizing about standing close to a massive spinning generator, but there is more.

In addition to generating electricity, these mechanisms provide another important grid function that doesn’t usually come to mind when power production is discussed. Power engineers call it grid inertia, which is simply resistance to change. A spinning mass follows the Newtonian laws of motion. In this case, we are interested in the law that says an object in motion tends to stay in motion, and a really big mass tends to certainly do that.

Why is that important? Well, inertia provides the transmission grid with stability and resiliency by resisting change. Think of the power station’s spinning generator as a flywheel (i.e., a mass rotating around an axis) that stores energy. The grid inertia these devices provide is needed by the transmission grid to ride-through network faults when they happen. In effect, the generator’s rotational energy performs like a shock absorber, but today’s generation mix is being challenged to provide that inertia.

Retiring Fossil-Fuel Plants

The modern grid has more renewable generation than ever before and that generation is replacing heavy rotating equipment like steam and gas turbines at an expanding rate. The US Energy Information Administration (EIA) forecasts, “Renewable energy will be the fastest-growing source of electricity generation in 2020. EIA reported, “They expect the U.S. power sector will add 23.2 gigawatts (GW) of new wind capacity in 2020 and 7.9 GW of new capacity in 2021.” EIA also expects utility-scale solar capacity to rise by 12.8 GW in 2020 and by 13.0 GW in 2021.

On the flip side of the widespread growth of renewable generation is the retirement of fossil-fuel power plants. One report said about 126 GWs of fossil generation capacity had been taken out of production between 2009 and 2018. The Guardian reported that in the first half of 2020 21.2 GWs of coal power plants worldwide were shut down and 5.4 GWs were in the US. These figures are eye catching, but they are also important to the operation of the power delivery capabilities of our smart grid.

It is important to keep in mind that the renewable generation being installed is intermittent and variable. It lacks the ability to tolerate network faults or handle frequency oscillations, which have caused experts to become concerned over reduced grid stability. There are, however, several technological solutions that have been in use on the grid for many years. In fact there is one that dates back to the early days of the grid and is experiencing renewal of interest. It is the venerable synchronous condenser.

The synchronous condenser is not a motor or a generator. It’s a synchronous machine that has similar characteristics to a generator. Some refer to the device as a relic of the past, but they need to look into the today’s adaptions. The basic device is proven, and when integrated with digital technology there is nothing old-school about the application. It has become a valuable FACTS (flexible AC transmission systems) tool for the industry.

When FACTS controllers were introduced to the grid, the newer power electronics based devices SVCs (static VAR compensator) and STATCOMs (static synchronous compensator), etc.) got all the attention. But over the years other devices were added to the FACTS controllers catalogue such as the synchronous condenser and series capacitors with thyristor controls.   

FACTS devices, in case anyone was wondering, were developed to solve technical problems with interconnected power systems. According to IEEE (Institute of Electrical and Electronic Engineers), these power electronics-based systems are designed to control one or more AC transmission parameters like voltage, impedance, phase angle, etc., which is where synchronous condensers come into play. They produce or absorb reactive power, and this is where it gets complicated.

The basic version is — real power delivers the energy consumed to operate the end customer’s equipment (lights, motors, etc.) and is measured in watts. Reactive power is the current flow needed to get the real power to the customer. It’s measured in volt amperes reactive (VArs). There is also apparent power, but let’s save that for another day - now back to synchronous condensers.

A Stabilizer

Synchronous condensers are large rotating machines, which provide the grid inertia, short circuit power, and reactive power for dynamic loads. Like most technologies, there are several names for the device, which is confusing, such as synchronous compensators, SCs, SynCons, synchronous capacitor, and several others. Keeping things simple we’ll stick with synchronous condensers. They have been installed on the grid for a very long time and there is a great deal of operational data available. They are dynamic sources of reactive power with voltage support capabilities, frequency stability, fault ride-through and fault support, which are well understood.

Typically, a synchronous condenser has a synchronous generator connected to the transmission gird through a step-up transformer. The synchronous generator is started and stopped by a variable speed motor that is referred to as a pony motor. The synchronous generator is brought to synchronous speed (grid frequency) and synchronized with the transmission network. At this point, the device performs like a synchronous motor with no load, which allows it to provide reactive power, inertia, and short circuit power to the transmission grid.

Several years ago, San Diego Gas & Electric (SDG&E) was faced with grid stability issues when the San Onofre Nuclear Generating Station was decommissioned, and California shifted to renewable generation. To mitigate the issues, SDG&E installed seven synchronous condensers on their network. The last unit was installed in late 2018 and at last report they were preforming as expected.

Terna S.p.A, the owner and operator of 98% of the Italian high-voltage power transmission grid, announced it has contracted with GE for two synchronous condensers and flywheel units for their Brindisi substation in southern Italy. Each unit will supply 500 MVar reactive power and 3,500 MWs inertia. There are a total of eight GE synchronous condensers either under contract or in operation on Terna’s system. These devices will supply up to 1,820 MVar of reactive power with a value of 10,500 MWs of inertia to stabilize the grid and support the integration of more renewable energy in the Sardinia region, Sicily, and southern Italy.

The state of South Australia (SA) generates more than half their electricity needs from renewable sources, which puts them are risk for a less stable power system. As a result, the Australian Energy Market Operator proclaimed a shortfall in system strength in SA. In 2019, the Australian Energy Regulator approved a $190 million investment by ElectraNet for four synchronous condensers to strengthen the SA power system. GE is installing the first two at the Davenport substation near Port Augusta, which was scheduled to be completed in late 2020 or early 2021. Siemens is installing the second two in the Robertstown substation and will be operational in the middle of 2021.

The United Kingdom is shifting from traditional generation to renewables and is working on new technologies to make this transition smoother. The Phoenix project has been developed to help the transition. It started in 2018 when Hitachi ABB Power Grids installed the world’s first hybrid solution in SP Energy Networks’, Neilston substation near Glasgow, Scotland. Recently the facility began a yearlong performance testing of the first-of-its-kind hybrid scheme. The scheme combines a STATCOM with a synchronous condenser, and hybrid controls. According to Hitachi ABB, the STATCOM operates in parallel with a synchronous condenser. They are connected to the bus via a three-winding power transformer. The system will inject or absorb energy into the network to maintain the voltage level within the required limits. In effect, it will provide a spinning reserve over a few seconds until other resources can be brought online.

This is an interesting time for the grid operators, renewable energy producers and regulators as the power system shifts to different types of power production methods. In addition to synchronous condensers, there are many technologies being explored to stabilize the transmission network. Some operators require “primary frequency response” from interconnectors. This has resulted in some noteworthy ideas and concepts such as synthetic inertia (energy injection), virtual inertia (grid-forming inverters), zombie power plants (repurposed generators) and some innovative grid following battery systems.

Whether it is reducing a carbon footprint or becoming carbon neutral, increasing renewable energy is changing the grid from both sides of the meter. Yesterday’s mechanisms infused with today’s digital technologies are making this possible. The surging demand for wind and solar produced electricity require flexibility and stability from the grid and synchronous condensers are critical elements.

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