Thorben Fohrmann and Bernd Niemann from Siemens Energy explain how to ensure the stability of power grids built for central power plants in an age of increasing distributed generation.
On January 8, 2021, at around 2pm, the Continental Synchronous Area unexpectedly split in two – with potentially serious consequences for all of Europe.
The cause was the outage of several network elements. It started in a substation in Croatia with the malfunction of a so-called busbar coupler, which helps connect various components in a substation, including overhead lines, cables, and electrical switches.
The effect of the split was twofold: the northwest area of the power grid ran low on power, with a corresponding decreased frequency; and the southeast area had a power surplus, meaning the frequency increased. Power outages for millions of people could have been next.
In the northwest area, around 1.7GW of interruptible services were being cut in France and Italy, while supportive power was being added from Nordic countries and the UK.
This ensured the deviation stabilised at 0.1 Hz below 50 Hz. Similarly, in the southeast, power was taken offline, reducing the frequency to an acceptable 50.2 Hz. Shortly thereafter, a few minutes after 3pm, both areas were resynchronised, and business as usual resumed.
It was the most serious near-blackout incident in Europe since 2006, when the lights went off for 15 million people after miscommunication regarding a planned power line disconnection in Germany.
Causing great alarm at the time, it was fortunately resolved expediently by transmission system operators, who were able to coordinate and react in real-time.
At any rate, there’s no guarantee that everything will go as smoothly next time.
This article is part of the ‘Future Energy Perspectives’ series, in which experts from Siemens Energy share their insights into how we can move towards a decarbonised energy system.
Enabling the energy transition
Today, the energy system continues to evolve. The changes are numerous: fossil power and rotating masses continue to be phased out and the share of renewables keeps rising, introducing volatility.
Whereas before the grid was dominated by large central power plants, power generation today is becoming more and more decentralised while increasing system complexity. Moreover, as renewable energy is often generated far from areas where energy is consumed, transmission distances increase substantially.
And finally, as energy demand continues to rise, even more pressure is put on the system.
The grid we have now was not designed for meeting these challenges. That’s why the grid needs to be safeguarded to make energy transition possible, particularly with the help of new technologies as well as serious investments.
Four areas for adjustment
There are four features of the grid that need to be adjusted given the new energy reality.
Firstly, maintaining the frequency of power transmission is essential. Secondly, voltage stability needs to be ensured. Thirdly, load flow needs to be stable.
And finally, black start capability would be crucial for grid recovery in case of a serious outage.
All these features need to be functional even in a changing environment, otherwise we won’t be able to deliver the energy needed, and the energy transition won’t happen.
The grid we have now was not designed for meeting these challenges. That’s why the grid needs to be safeguarded to make energy transition possible, particularly with the help of new technologies as well as serious investments.
Gaining time for next steps
Maintaining frequency control means keeping the balance between load and demand on a grid – the ‘heartbeat’ of the grid, if you will.
Globally, the grid frequency is mostly balanced to either 50 Hz or 60 Hz, depending on the regional market. It must be kept within specific tight limits, even in the event of a frequency disturbance.
Four important steps should be taken at different times to keep the frequency stable if a power line gets disconnected, a power plant fails or electric equipment malfunctions, such as what happened in Croatia.
The first response – within the first ten seconds of a disturbance – would be the inertial response of the system, which in the past was ensured only by the physics of the rotating equipment, such as by turbines and generators.
But today, fewer rotating machines result in shrinking instantaneous reserves, which increases the risk of exceeding critical frequency levels. Grid operators are thus forced to keep power plants in operation to preserve the instantaneous reserve or invest in additional primary reserve.
Additional measures include rotating grid stabilisers as well as power electronic-based solutions, namely supercapacitors and batteries that emulate the behaviour of rotary masses.
Pushing frequency
Then as a second step, after ten to 30 seconds, the primary frequency response should start influencing local grid frequency.
In case of low frequency, it usually generates and feeds more power to the grid. In case of high frequency, it reduces power output or increases consumption. Li-ion battery energy storage solutions can also be part of the primary response.
However, this might still not be enough to reach the targeted frequency. And that’s why, as a third step, i.e., within 30 minutes after a frequency disturbance, an energy management system should direct grid-wide generators to push the frequency towards the desired value.
In the Continental Synchronous Area, around 3000MW are kept in constant reserve to address any major frequency imbalances.
As a fourth and final step, if a longer imbalance occurs between supply and demand in a grid, reserves, such as additional power stations and storage facilities, can be used to contribute more energy or remove it from the grid.
Power electronics
But frequency is obviously not the only thing to keep in check. Just like frequency, voltage needs to remain at a certain level to ensure consistent power quality.
This is crucial to maintain, as power often travels long distances and renewables’ in-feed is intermittent.
The main lever here is reactive power, which usually comes from synchronous generators in conventional power plants. However, with the share of solar and wind steadily increasing, other means are needed to address voltage, especially power electronics.
One example is static synchronous compensators (STATCOM), which are based on semiconductors called IGBTs. They help operators avoid blackouts by providing the reactive power needed within milliseconds to stabilise voltage.
They do so by acting like an active voltage source, while increasing the transmission capacity of power lines. As a result, high power demand can still be met by centres of renewable energy generation often located far from areas where energy is needed.
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Load control
Next to frequency and voltage control, load management is another major building block for grid stability. It’s especially challenging, as the increase in renewables runs the risk of overloading existing transmission lines.
A solution to control the load flow is to use Unified Power flow Controller (UPFC) based on power electronics. It can expand the transmission capacity of lines by compensating the electrical impedance of the overhead line.
If overload occurs, another remedy would be redundancy, whereby power is redirected to parallel lines to distribute power more evenly.
Batteries to ease congestion
But to safeguard the grid, we also need other flexible devices to actively control the power flow. Energy storage solutions help with the so-called transmission and distribution congestion relief.
Batteries can help with transmitting electricity over long distances, such as power generated by wind coming from the northern part of Germany and transmitted to the southern part. With large amounts of energy being transported, the risk of overloading lines is high, which means energy generation in the northern part may be curtailed.
Moreover, as energy generation is decreased, more energy may be needed towards the southern part to meet demand, resulting in the need for additional infeed.
Battery storage facility can help avoid expensive redispatch measures by absorbing surplus energy and dispensing it whenever and wherever necessary.
And in case instabilities in the grid cause a blackout, batteries can also help by delivering the necessary jolt to turn lights back on.
Finally, during ‘dunkelflaute’ – a period in which little to no energy can be generated due to insufficient wind and solar power – long-duration energy storage solutions, such as thermal and compressed air energy storage, hydrogen or pumped hydro may help to keep energy flowing.
Smart controls, financial incentives
Of course, all this would not work without smart control systems. As an example: part of the complexity in the energy grid is due to the high number of power-generating assets, such as households with solar PV systems.
One solution for meeting this challenge is pooling these small power producers together with the help of smart controls to form a virtual power plant.
Finally, all of this wouldn’t work without financial incentives. That’s why utilities as well as TSOs are being asked to offer reserves for the grid. At the same time, a framework needs to be in place ensuring that utility operators get paid fairly for adding capacity to the grid even in times when it’s not being used.
For example, in Europe, 11 TSOs form the Frequency Containment Reserve (FCR) cooperation. Among their various responsibilities, they manage a marketplace, where providers offer positive as well as negative reserve.
Which provider gets to do so is determined by daily auctions. They are then listed in a so-called merit order, with capacity cost determining which provider comes first. And in case of a frequency deviation, each member can call upon these resources.
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Looking ahead
The near-blackout of January 2021 started with a malfunction of a busbar coupler in a substation in Croatia.
Beyond the inertial and the primary response, reserves were used for the automatic generation control and the reserve deployment. This decisive step was determined by the marketplace within the FCR just the day before.
Together with other measures, such as rerouting load flow, the energy supply of the complete region was fortunately not seriously endangered.
But as the energy system continues to change, it’s obvious that other incidents can occur at any time. That’s why it’s important to have measures in place to keep the grid stable. This way, we can minimise the risk of serious power failures.
ABOUT THE AUTHORS
Bernd Niemann is an expert in AC grid stabilization at Siemens Energy. He has a strong background in System Engineering for FACTS and is now responsible for developing the Grid Stabilization business by focusing on current and future customer needs.
Thorben Fohrmann is an expert in the field of energy storage. He develops go-to-market strategies including sales enablement and strategic marketing, and he also drives strategy formulation, including competitor benchmarking and market analyses of energy storage technologies.
