Balancing a renewable grid: What are the options?

By Archie Robb

Over the past decade or so, the dominant trend has been the retirement of coal plants and the steady advance of renewable sources of generation. The next wave appears to be the phasing out as many natural gas-fired facilities as is feasible as more wind and solar resources are deployed.

But there has been an unintended consequence in this headlong rush to displace rotating generation assets – grid instability. The steam turbines and gas turbines of power coal plants, combined cycle and natural gas peaking facilities play a vital role in terms of grid inertia, stability, and the provision of reactive power in the form of VARs (Voltage Ampere Reactive).

In the UK, for example, the government has committed to phasing out all coal-fired power generation by 2025. Over the past decade, the nation has installed around 20 GW of renewable power. As a result, just over a third (37.1%) of the UK’s electricity comes from renewable sources with plans to install a further 40 GW of offshore wind over the next decade. As more wind and solar floods onto the grid, instability and inertia issues will increase.

“Renewable power is connected to the grid electronically rather than directly as a large centralised power station would be,” said Mark Tiernan, Head of High Voltage Substations United Kingdom at Siemens Energy. “As a result of the shift away from coal, there are fewer large spinning turbines on the grid, and this has led to a reduction in the amount of inertia in the system. The loss of synchronous gas turbine and steam turbine generators leads to system instability in the form lower system inertia.”

Electrical Gear

A variety of approaches are springing up that aim to provide such services in order to maintain and accelerate the pace of renewable adoption. Traditional electrical solutions to this problem include capacitors, static VAR compensators, and static compensators. Capacitor banks are typically installed at electrical substations. They consist of shunt capacitors. They are relatively cheap, reliable, and easy to install. But disadvantages include their large footprint, and the fact that they can only supply reactive power, they cannot absorb it. When load rapidly increases and voltage drops, the effectiveness of capacitors diminishes.

Static VAR Compensators (SVCs) are basically electrical switches. They consist of shunt capacitors and reactors and offer a greater degree of voltage control than simple capacitors. They can absorb and supply reactive power, but struggle in the face of voltage instability or collapse.

Static synchronous compensators (StatComs) make use of sophisticated power electronics rather than capacitors and reactors. They provide a much faster response time (microseconds) and take up less space. But are pricey compared to more basic equipment. Examples include American Superconductor’s Dynamic VAR (D-VAR) systems, S&C Electric Company’s Purewave DStatCom, and Siemens Energy’s SVC Plus. 

SVC Plus combines a StatCom and multilevel converter technology. The guts of the system comprise a collection of electrical components such as insulated gate bipolar transistors (IGBT), reactors, capacitors, and AC power transformers. It can quickly inject inductive or capacitive power to stabilize transmission systems and reduce the risk of voltage collapse and blackouts. This Siemens Energy unit is about half the size of a conventional SVC. 

German transmission system operator Amprion commissioned Siemens Energy to provide two SVC Plus systems to stabilize the German grid. The plants are for Polsum, North Rhine-Westphalia and Rheinau, Baden-Württemberg. They provide a reactive power range of +/- 600 MVAR and keep the grid voltage in a stable range. Overall, transmission operators calculate that the German grid needs up to 28 GVAR to provide enough stability and inertia.

Italy, too, is adopting this technology. Terna. S.p.A has ordered two SVC Plus systems. They will contribute to interconnections between Italy and Montenegro and mainland Italy and Sardinia. Terna has two similar systems being installed in Italy’s Marche region. They will go online gradually between late 2021 and mid-2022.

How the Siemens Energy synchronous condenser works.

Synchronous Condensers  

Synchronous condensers are another way to address grid instability issues. Once again, there are a variety of systems on offer. Siemens Energy and GE offer competing electrical systems.

The Siemens Energy unit comprises a synchronous condenser to provide inertia to strengthen the grid, short circuit power for reliable operation, and reactive power for voltage control. In essence, the synchronous condenser is a large piece of spinning machinery made up of a generator and a flywheel. When connected to the grid, it provides the inertia by spinning continuously in sync with grid frequency. Thus, it contributes to the stability of the system, dampening any fluctuations in frequency, just as car shock absorbers dampen a bump in the road. The flywheel is a large wheel that adds additional mass for greater system inertia. It is effectively a means of substituting a flywheel for the rotating mass of a gas or steam turbine.

“By coupling the fly wheel to the rotating mass of the generator’s rotor, it provides the short-circuit contribution and enlarge the necessary inertia,” said Tiernan. “In this way, they will help stabilizing the networks frequency.”

The synchronous generator is 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 has reached operating synchronous speed, it is synchronized with the transmission network, and the machine is operated as a motor providing reactive and short-circuit power to the transmission network.

The UK’s National Grid Pathfinder program aims to provide plenty of short-circuit power, particularly in locations in Scotland and Wales. Siemens Energy was awarded three projects as part of this program. Work has begun at a site at Rassau, Ebbw Vale in Wales for Welsh Power. This rotating grid stabilization technology is being installed at the site to manage grid stability. It will be up and running before the end of the year.

“Within 15 minutes of an instruction, our facility can provide approximately 1% of the inertia needed to operate the grid safely with zero emissions,” said Chris Wickins, Director of Grid Services at Welsh Power.

A similar system is being supplied to the Electricity Supply Board (ESB) in Ireland for the Moneypoint power station located in County Clare. ESB is turning the site into a green energy hub, where a range of renewable technologies will be deployed over the next decade.

“Due to the intermittency of wind energy in particular, grid stabilization technologies have an increasingly important role in a successful energy transition,” said Paul Smith, Head of Asset Development at ESB Generation and Trading.

GE Steam Power, meanwhile, is touting its own synchronous condensers and flywheel system. It sold two such units to Terna for the Brindisi substation in Italy. Each will supply up to +250/-125 MVAr of reactive power and 1750 MW inertia. They are being installed along the transmission system to keep the power flowing consistently. GE has an additional four 250 MVAr synchronous condenser units under execution with Terna in the Selargius and Maida plants in Sardinia and Calabria. Additionally, GE has delivered two 160 MVAr synchronous condensers for Favara and Partinico Terna Substations in Sicily that have been running since the end of 2015. That adds up to 1,820 MVAr of reactive power for Italy’s grid.

A typical synchronous condensing plant layout consists of one or two synchronous condensers units with flywheel in parallel, step-up transformers, generator circuit breakers, all the electrical and mechanical auxiliaries and balance of plant including the protection and controls systems, monitoring and diagnostic systems. Courtesy of GE.

“Units consist of either new electrical rotating equiment or existing generators reconfigured to perform as reliable grid stabilizers, that means to stabilize the voltage of the grid,” said Chris Evans, Head of Product Management, GE Steam Power. “Flywheels are an add-on feature for additional inertia that can be delivered at the time of the construction of a new plant or added later on during the life cycle of the plant, which instead serve to stabilize the frequency of the grid.

Existing Generators for Synchronous Condensing

The various systems showcased so far all do the job. But a less capital-intensive approach is available by converting old steam and gas turbines into synchronous condensers. There are many power plants in existence that have aging turbines available. Some have already been decommissioned, and many are running at much lower capacity that in previous years as renewable resources take on a bigger share of power supply. Inevitably, more and more of these units will either be decommissioned or gradually phased out.

Conversion of existing generators to provide synchronous condensing falls into two categories. One is for the machine to be used for peaking power and synchronous condensing by incorporating a synchro self-shifting (SSS) clutch into an existing turbine generator set. Alternatively, an existing turbine generator set such as decommissioned coal plant steam turbine generator can be rapidly converted to a synchronous condenser by removing the turbine and adding an acceleration drive with an SSS clutch (see lead image)

The turbine, or acceleration drive in the case of a generator-only application, brings the generator up to speed. Once the generator synchronizes with the grid, the turbine or acceleration drive disconnects from the generator and shuts down. The generator then uses grid power to keep spinning, constantly providing leading or lagging VARs as needed.

As well as steam turbines and gas turbines, such conversions can be done for reciprocating engines. The clutch acts by completely disengaging the prime mover from the generator when only reactive power is needed. When active or real power is needed, the SSS clutch automatically engages for electric power generation. This enables the unit to absorb or supply reactive power to the grid for voltage control purposes by running the generator as a synchronous motor uncoupled from the gas turbine. New gas-fired power plants being built can also be configured to operate in this way.

“There are significant savings in the fact that an existing generator is in position, connected to the transmission system, and already in working order with controls,” said Dave Haldeman, SSS Clutch. “Additionally, this approach provides the system for a backup power or peaking power, which complements the renewable power when required.”

The Commonwealth Chesapeake Power Plant in rural Virginia consists of 7 GE LM6000 gas turbines, installed about 20 years ago. They provide power sporadically, depending on the needs of the grid operator. As a result, four of them are equipped with clutches.

These generators are outfitted with clutches that can disconnect from their turbines to enable them to operate as synchronous condensers. In this case, the grid operator pays the plant to have the generator synchronized and spinning but not connected to the power turbine. That offers grid support. Once power is required, it can be on the grid within 10 minutes to respond to generation or transmission outages elsewhere in the network. Control software is used to bring the turbine rapidly up to near-synchronous speed in order to engage or disengage the turbine. When disengaged, the generator continues to spin.

Four units at this U.S. power plant were outfitted with clutches to enable GE LM6000 turbines to provide rapid standby power as well as reactive power support.

Haldeman sees a place for both purely electrical synchronous condensers, as well as those utilizing old engines and turbines. As more renewables are added, the demands for inertia and grid stability will only accelerate.

From a purely economic perspective, money can be saved by utilizing machinery that is already in place. An inexpensive retrofit can add a clutch in a couple of weeks. Transmission lines, switch gear, other electrical gear, as well as permitting are already in place. The money saved can then be used to upgrade other areas of the grid or be invested in more wind and solar projects. Further, existing generators tend to be installed near load centers. In most cases there are already in a location where they can support the need for reactive power and provide the resulting voltage support. 

As the turbine is not running in synchronous condensing mode, there is no fuel burn and therefore no emissions.  There is a general tendency to paint all sources of emissions with the same brush. But there is a big difference between an aging coal plant and a natural gas fired turbine in terms of emissions.  

“Peaking gas turbines have an important role to play in maintaining grid stability by providing inertia, and reactive power support,” said Haldeman. “These natural gas peaking units can provide critical standby power when the renewables are at reduced levels, such as a deep freeze or some other extreme weather event. Otherwise they offer synchronous condensing and voltage support which will be in high demand as a greater percentage of renewable assets come online.” 

About the Author

Archie Robb is a writer based on Southern California who covers business, energy and technology.

Source: Renewable Energy