Changing energy mix and its impact on grid stability

2 Coauthored by Swiss Re Corporate Solutions and ETH Zurich Changing energy mix and its impact on grid stability

Content

Introduction 3

Challenges for grid stability from renewables integration 4With the move to renewables and retirement of thermal baseload generation plants (coal, gas, nuclear), what are the top challenges facing grid stability, and how are they being addressed? 4

What is the impact of inverters on overall system strengths? 5

How do the top challenges differ by country? 5

With the move to more wind and Photovoltaics, the inertia in the systems will be reduced. To what extent is this shift impacting the grid stability, and what are the possible mitigation methods? 6

Managing the challenges 8Who is responsible for managing reactive power, short-circuit current, and frequency stability on the grid? 8

How are they addressing these growing issues? 8

Can the uptake of nuclear energy smoothen out the negative effects on grid quality from the decommissioning of coal plants? 9

Will the risk of power blackouts increase? 9

Conclusion and Future Outlook 11

Acronyms 12

Disclaimer 13

Contributors 14

References 15

Coauthored by Swiss Re Corporate Solutions and ETH Zurich Changing energy mix and its impact on grid stability 3

Introduction

The global energy sector is transforming rapidly towards less carbon-intensive energy systems. Many developed countries have set renewable energy targets that are backed by policy directives.

In the early stages of this transition, renewable energy assets had little impact on grid stability. Due to their low penetration level, they could be either connected or disconnected as required.

However, with a growing share of renewable sources in the energy mix, further integration of renewables poses an increasing challenge to power system stability due to its impact on volatility of power flow patterns, reactive power sufficiency and system inertia.

Given the increasing share of renewable energy in the energy mix, we expect that managing grid stability becomes a challenge going forward. Grid stability incidents have already started to emerge, for example, in Australia [2], where the blackout in South Australia on 28 September 2016 is sometimes referred to as the first known blackout linked to high penetration of renewable energy [3]. Even though it is important to stress that the blackout was not caused by renewable energy sources (RES) only, but also by various damages (e.g., high-voltage pylons) due to a one-in-fifty-year storm, the event made a case for more resilient electricity grids with a high share of renewables. In their final report, the Australian Energy Market Operator (AEMO) concluded that to avoid such events in the future, operators need to increase system inertia, the frequency of control services and to strengthen the overall system [4]. The Australian example highlights the challenges to overcome as we transition towards a decarbonised electricity supply.

In this paper, Swiss Re and the Reliability and Risk Engineering Laboratory at ETH Zurich discussed a series of questions on the grid stability topic. The discussion addressed the key challenges from the increasing share of RES within the energy mix, how they can be managed, and what system-level solutions are available.

Managing system reactive power is how operators ensure that voltage levels in the system remain within a given range, above or below nominal voltage levels. System inertia refers to the kinetic energy stored in large generators’ rotating mass, such as those found in fossil-fuel based power plants. System inertia is vital for maintaining a stable frequency level. Steady and predictable power flow patterns facilitate the successful response of system operators against unanticipated disturbances [1]


Challenges for grid stability from renewables integration

With the move to renewables and retirement of thermal baseload ge-neration plants (coal, gas, nuclear), what are the top challenges facing grid stability, and how are they being addressed?

RES are expected to account for over half of 2021’s new energy generation portfolio. Almost half of this uptake is expected to occur in China, followed by the United States of America, the European Union, and India [5].

With increasing penetration of RES are the retirement of thermal baseload generation plants, three new challenges arise to maintain grid stability [6]:

1. Ensuring sufficient flexibility for power system operations and supply2. Tackling increased operational complexity of the power system3. Integrating inverter-connected devices.

Ensuring sufficient flexibility while managing increased operational complexityPower system flexibility is “the ability to adapt to dynamic and changing conditions". Examples of such ability include balancing supply and demand by the hour or minute, or deploying new generation and transmission resources over a period of years [7].

Flexibility is particularly desirable given the pace with which the electric power system is changing and that more RES and other smart energy technologies (e.g., electric vehicles, at different electricity voltage levels) are being integrated.

On a transmission level, flexibility can come from the dispatchable energy generation plants, such as gas-fired and hydroelectricity, or it can come from the power exchange with neighbouring countries. Depending on the potential of hydropower and the degree of connectivity between neighbouring countries, system operators can decide the best option to pursue.

Good connectivity between neighbouring countries requires smooth coordination among the transmission system operators (TSOs) and better usage of the interconnectors with reduced safety margins to manage potential overloads. Furthermore, new transmission infrastructure will also be required [6].

At a distribution level, further flexibility can be provided by Demand Side Management (DSM) and, potentially, bidirectional electricity flows between the grid and local energy storage, such as battery systems and electric vehicles. In addition, coupling the power sector with other energy sectors (e.g., heat and gas sectors) provides flexibility options both at the distribution level [8] and at the transmission level [9], [10].

Energy transition is expected to affect power system operations [6]. With the integration of large share of RES, in particular rooftop solar Photovoltaic (PV) it can be anticipated large power injection from distributed generators during peak load hours. This will lead to a few challenges. Firstly, the large power injection will affect the net load patterns and, as a result, requires adjustment to the current peak load providers’ operation (hydro and gas). Secondly, conventional storage units such as hydro dams and pumped storage may need to adjust their operational behaviour significantly. Thirdly, the increase in variable generation will demand conventional units (flexibility providers) to react more rapidly to balance energy generation and demand. Last but not least, increasing power generation from distributed sources, coupled with the flexibility potential from distributed batteries and DSM, will require more coordination between Transmission System Operators (TSOs). and Distribution System Operators (DSOs). The analyses performed in the Nexus-e study [6] at ETH

Interconnectors are transmission lines connecting the power systems of neighbouring countries and serve a crucial role in enabling long-distance power transfers in interconnected power networks.


Zurich also show an increase in electric power exchange between neighbouring countries and thus implies a need for more coordination among power system operators. Overall, the transition is expected to increase the challenges of future power systems operations

Integrating inverter-connected devicesInverter-based technologies, such as PV, wind, fuel cells, microturbines, batteries, and electric vehicles, will play a crucial role in energy transition [11]. Unlike traditional rotating generators, these technologies rely on so-called inverters to transform the direct current (DC) electricity into alternating current (AC) electricity. However, integrating inverter-connected devices poses a significant technical challenge. The inclusion of such devices and the simultaneous decommissioning of synchronous rotating generators has two main consequences for the system stability: the decrease of system inertia and the reduction of short-circuit power.

Lower system inertia will reduce a system‘s ability to respond to disturbances. A lower short-circuit power will increase the impedance seen by generators and thus reduce system stability. Additionally, a reduction in short-circuit power will lead to a higher magnitude of voltage dips, impacting the system further. For example, a lower short-circuit power might cause commutation failures on High Voltage Direct Current (HVDC) links and impact the operation of inverter-connected generators [12]. The North American Electric Reliability Council (NERC) has also expressed concerns about the unexpected disconnection of large amounts of inverter-connected devices. Inverters typically disconnect in the event of abnormal voltage or frequency deviations to protect the equipment. However, these inverter disconnection rules can trigger cascading failures of the power transmission assets and change the dynamics of a power system, thus reducing system stability [13]. Power system stability issues stemming from inverter-connected devices is further noted by the European Network of Transmission System Operators (ENTSO-E) [12], [14].

What is the impact of inverters on overall system strengths?

The output current of generators with power electronic converters is maintained within a constrained threshold to protect the semiconductor switches against overloading. However, this current limitation results in reduced system strength, which is often represented in terms of short-circuit power. A weak network (i.e., a network with a low short-circuit power) faces several challenges as enumerated by International Council on Large Electric Systems (CIGRE) and NERC [22], [23]. The system is more susceptible to voltage instability in the face of generation and demand fluctuations. Inverter-based generators have reduced fault-ride-through capability when integrated into weak networks, i.e., they are less resilient to the faults occurring in the system. Weak networks are often marked by remote renewable generation that must be transmitted over long distances; hence, a potential issue in such networks is the overloading of transmission assets. The low levels of system strength may also introduce risks to protection systems [24] and increase relays’ trip time [25].

Installation of synchronous condensers can increase system inertia and strength. The generators of phased-out conventional power plants can be retrofitted for this purpose.

How do the top challenges differ by country?

The challenge to integrate RES while maintaining grid stability differs significantly by country and region due to the characteristics of their existing energy generation mix, geographic variations, and the existing/potential degree of regional cooperation [44]. The variability introduced by RES can be offset by having flexible, dispatchable energy sources, such as hydropower and gas-fired generation plants. The benefit of having these plants is that they may “balance” any difference between electricity supply and demand as they develop. For example, Swiss hydroelectricity production is highly valuable to the broader European network, which is more dependent on


Challenges for grid stability from renewables integration

highly variable solar and wind production. However, the benefit of these dispatchable electricity sources depends not only on their share of electricity mix compared to that of variable RES, but also on the overall interconnection of the electricity network. Australia, for example, has previously struggled with maintaining stability [3], [4] despite having a high share of natural gas in their domestic electricity production portfolios [15]; other countries, like Italy and China, have simply “curtailed” (temporarily halted) RES production [16]. The age and carbon intensity of the existing electricity generation fleet will also determine the urgency with which the national electricity generation mix ought to change, possibly adding pressure to integrate variable RES.

The challenge of maintaining grid stability is further dependent upon the geographic characteristics of the country in question. Countries like Norway, Switzerland, Canada, and Brazil can build large hydropower facilities due to their mountainous terrain and temperate climates. In contrast, countries in the Middle East, North Africa only have a limited potential, and projects often have a significant impact on downstream countries.

Proximity and connection to other countries can also be beneficial for managing national grid stability. A prime example is Denmark, which meets more than 45% of domestic electricity demand by wind and solar production, and benefits from extensive cross-border trading to meet its need for flexibility [17]. Increasing network connections, such as through high voltage overhead lines or subsea cables in islands, can help manage grid stability challenges. Countries (like UK for example) have significantly increased their interconnection capacity to France, Belgium and Norway over the last years, with more interconnection capacity still being constructed [18].

Country-level challenges may also be mitigated depending on the degree of regional cooperation, both in terms of regulation and integration of the power systems. The level of cooperation varies enormously between different regions in the world. An example of a trend towards strong cooperation and market integration is the European Union, which began with the Directive on common rules for the internal market in electricity in 1996. By establishing the Agency for the Cooperation of Energy Regulators (ACER) through the Third Energy Package in 2009, the EU has established the basis for market integration of the European countries. Since then, several network codes and guidelines have been developed, shaping the transition towards an integrated electricity market [19]. Even though there are still obstacles that need to be overcome, the European nations already benefit from their geographic proximity due to a regulatory framework for technical cooperation [20]. By contrast, the US Texan electricity grid is poorly connected to other regions for historical reasons, making it more difficult to maintain functionality when local disruptions occur [21].

With the move to more wind and Photovoltaics, the inertia in the systems will be reduced. To what extent is this shift impacting the grid stability, and what are the possible mitigation methods?

Reducing inertia threatens power system stability. System inertia responds immediately to sudden changes in the frequency level, for example, due to an unforeseen outage of a generator. Therefore, system inertia helps bridge the time until the system responds to the unforeseen change [1].

Even though inverter-connected generation can theoretically provide virtual inertia, such approaches have yet to be tested and implemented [26]. One of the possible mitigation actions consists of deploying different forms of non-synchronous renewable generation and energy storage such as hydropower or pump storage capacity or flywheels, which provide rotary turbine mass and fast reactions to power imbalances [27]. The optimal percentage of dispatchable generation that needs to be readily available as a backup to ensure grid stability and supply adequacy depends on the entire generation mix, i.e., the availability of hydropower generation


and distributed flexibility providers. Furthermore, it depends on the grid configuration, the interconnections and the trade with the neighbouring countries. This optimal percentage is identified by the transmission system operator, who has the mandate to ensure electricity supply at the national level.

With increasing PV and wind integration, reduced inertia is becoming an important issue in many power systems worldwide. The situation can be critical in countries with lower transmission capacities (long transmission distances, little meshing, limited capacity), such as Australia [28]. In such cases, improving the interconnection can also mitigate the effects of reduced inertia. Finally, improving the fault ride-through of wind generation (which does provide some inertia but needs to stay connected if the grid frequency is disturbed by a fault) could be an additional puzzle piece to mitigate inertia reduction [28].


Managing the challenges

Who is responsible for managing reactive power, short-circuit current, and frequency stability on the grid?

The responsibility for managing the reactive power, short-circuit current, and frequency stability resides with the system operator of the transmission grid. In Europe, this is the task of TSOs, who build and maintain the high voltage network and are responsible for its operation, procuring ancillary services and ensuring grid stability. Typically, TSOs operate on a national or regional basis, with most European countries having one TSO managing the entire country’s grid. By contrast, the United States of America has several entities managing the high-voltage electricity grid on a sub-national basis. Depending on their size, they are referred to as independent system operators (ISOs), or regional transmission operators (RTOs). Unlike their European counterparts, these entities do not own, but merely manage the electricity grid, and operate energy as well as ancillary services markets [29].

How are they addressing these growing issues?

Ensuring sufficient reactive power, frequency stability, and avoiding short-circuit currents are all key components of a well-managed grid. These technical issues are to some extent affected by the integration of RES, and are addressed on different time scales [6].

Reactive power management is used to control the voltage level of the grid and is therefore a crucial element for maintaining grid stability. Currently, reactive power is supplied mainly by synchronous generators, namely traditional rotary generators like coal, gas, and nuclear power plants. With the phase-out of these large synchronous generators, other sources are needed to supply the reactive power demand and maintain voltage stability. Wind and solar plants have the technical potential to provide reactive power management and can fill this need. Therefore, TSOs should include these sources in the provision of reactive power to ensure voltage stability, as is currently done in Germany [30]. The German grid code requires that the RES units operate up to a power factor of 0.95 and 0.90 for under- and over-excited operation, respectively [31]. A power factor of 1 means no production of reactive power, while a power factor of 0 indicates 100% reactive power, and no active power is produced. Alternatively, reactive power can be procured from non-power generating units with a constant power factor of 0 [32], [33], e.g., synchronous condensers, passive shunt compensators (i.e., fixed or switchable capacitors, filters, and reactors), static shunt compensators, and series line compensators (e.g., series capacitors). Some of these latter measures are already commonplace but may need to be expanded to meet the growing need for reactive power.

TSOs must also maintain a stable grid frequency by balancing generation and load at all times. The TSO maintains a stable frequency by activating power reserves in case of a deviation from the target frequency of 50Hz [34]. It is expected that the required amount of reserves will increase with the integration of RES [6], but it also depends on the remainder of the generation mix

The reasons for short-circuits are internal (failure of insulation) and external initiators (such as inclement weather events, foreign bodies, like branches, making contact with the electricity line). The TSOs regularly maintain their assets (transformers, overhead lines, cables, switchgear) to avoid equipment failures. Furthermore, they also maintain the vegetation corridors around the power lines to avoid branches coming into contact with the conductors. Recently, TSOs have been exploring machine learning methods to detect anomalies in power grid assets that will eventually lead to failures.

On a final note, there is widespread understanding that some of the responsibilities of the TSO may be shifted or complemented by services offered by their local, lower voltage level counterparts, the Distribution System Operators (DSOs) [45], [46]. The interest in having DSOs take on greater responsibility in managing the electricity system stems from the fact that many changes to the system are occurring at a


consumer level [35], [36]. Residential solar PV, adoption of smart appliances, and increasing sales of electric vehicles are all good examples. Incorporating these consumer technologies has a distinctly local impact, hence the suggestion towards managing them on the same scale. Thus, the role of DSOs will have to change, requiring them to take a more active role in the management of the electricity system. This change can already be observed in Germany, where the “Redispatch 2.0” regulation requires the DSOs to participate actively in the redispatch process from October 2021. This is a significant change from former times, where organising redispatch measures was the sole responsibility of the four German TSOs [37].

Can the uptake of nuclear energy smoothen out the negative effects on grid quality from the decommissioning of coal plants?

The role of nuclear power in the decarbonisation of the electricity grid is highly debated. However, nuclear power can contribute to a decarbonised electricity supply, and it is currently the world’s second-largest low-carbon electricity source after hydropower [38]. Whether nuclear power can help manage the negative effects of coal phase-out is relevant in countries like the UK, France, Finland, Russia, India, China, and South Korea, where about 40 nuclear reactors are under construction.

Generally, nuclear plants are base loaders, i.e. they typically run at full capacity most of the time. Hence, they are likely to reduce the residual demand that renewable electricity satisfies when online and provide some weather-independent electricity supply. Although nuclear power plants are baseload units, they can change their power output to adapt to changes in demand [39]. The rate of change in the output of nuclear plants ranges from 1% to 5% per minute [40]. This allows nuclear power plants to provide frequency control services, including primary, secondary, and tertiary reserves [41]. Good examples of this mode of usage are the nuclear power plants in France [42]. Thus, it can be assumed that nuclear power plants can partly compensate for RES variability and thus smoothen the decommissioning effects of coal-fired power plants. However, the cost at which nuclear power plants can supply reserves may be higher than the typical providers (coal, hydro, gas).

The high availability of nuclear power, irrespective of most weather conditions, can be valuable during low solar and wind availability times. The main weather-induced unavailability of nuclear power plants is expected to occur when high atmospheric temperatures and droughts are present, such as in a heatwave. In this case, the cooling of nuclear plants can be affected, leading to a reduction in power output or even a complete shut-down [43].

Will the risk of power blackouts increase?

Scenario analyses on the Swiss power system transition show how the risk of cascading failures changes in 2020‒2050 [6]. Cascading failures have the potential to split the power transmission network and lead to electrical instability and blackouts. This study considers the phase-out of nuclear generation and significant increase of RES in the system, affecting the flexibility providers’ operations. Furthermore, under such scenarios, the utilisation of the power grid changes. Although, on average, the loading of the grid decreases, there is an increase in extreme asset loading cases and an increase in the utilisation of the interconnectors. In general, the increase of extreme operating conditions and the decrease of safety margins for power overloads can bring unprecedented challenges for the system operators, which may require the control room to react in a very short time frame. The analyses show that, with multiple failure events, the demand not served increases compared to the demand not served of the 2015 reference year. However, the system security can be restored to a level similar to 2015 with transmission system upgrades. Furthermore, the study shows that in scenarios where the amount of battery energy storage systems (BESS) and DSM increase, i.e., more distributed flexibility is available, the risk of cascading failures decreases. At the moment, these


Managing the challenges

analyses are subject to pronounced uncertainties and assumptions, and should be considered as indicating general trends rather than providing exact quantifications.

On a different note, the NERC and the ENTSO-E have voiced concerns that large amounts of inverter-connected devices can inject unprecedented common cause failure modes in power systems [12], [13], [14]. This is often the case when new technologies penetrate massively in traditionally conservative businesses such as the electric power sector..


Conclusion and Future Outlook

Maintaining grid stability is key to ensuring integrity of the electric power system. As the share of renewable energy sources in the overall electricity production mix continues to rise, maintaining grid stability becomes more challenging. Simply put, power grids relying on higher contributions from variable renewable energy sources require a greater degree of operational flexibility than those relying on dispatchable energy sources, such as thermal, hydroelectric, and nuclear generators. From this perspective, three main challenges arise for integrating renewable energy sources, namely:

1. Ensuring sufficient flexibility for power system operations and supply2. Tackling increased operational complexity and reduced centralised control of the

power system3. Integrating inverter-connected devices.

The scope of the challenge and associated mitigating strategies depend strongly on context. The existing generation mix, geography, and regional cooperation & power grid interconnection are all contributing factors. Several possible future developments can greatly influence the ease and degree to which renewable energy is integrated into the electric power supply, including:

Significant expansion of Demand Side Management Sector coupling, e.g., via electrification of heat and transport [47] Widespread adoption of vehicle-to-grid (V2G) charging and discharging

technology [47] Cost and technological breakthroughs allowing for a wider deployment of power-

to-X technologies, especially power-to-hydrogen [47]–[49] Reconsideration of nuclear power as an acceptable low-carbon energy generation

source [50]

Notwithstanding these potential developments, an array of strategies for maintaining grid stability already exists. These strategies include increasing the electricity network interconnection, ensuring a well-functioning power market, ensuring sufficient reactive power, managing renewable energy at a local level, and maintaining network assets. Moving forward, system operators must prepare for potential new investment and maintenance priorities, increased collaboration with fellow operators, or the need to adopt new business structures. Employing all of these strategies and getting ready to adopt new developments will be essential to further integrating renewable energy sources into the electric power system.


Acronyms

AC: Alternating Current

ACER: Agency for the Cooperation of Energy Regulators

AEMO: Australian Energy Market Operator

BESS: Battery Energy Storage System

CIGRE: International Council on Large Electric Systems

DC: Direct Current

DSO: Distribution System Operators

DSM: Demand Side Management

ENTSO-E: European Network of Transmission System Operators for Electricity

HVDC: High Voltage Direct Current

ISO: Independent system operators

NERC: North American Electric Reliability Corporation

PV: Photovoltaic

RES: Renewable Energy Source(s)

RTO: Regional transmission operators

TSO: Transmission System Operator


Disclaimer

The entire content of this report is subject to copyright with all rights reserved. The information may be used for private or internal purposes, provided that any copyright or other proprietary notices are not removed. Electronic reuse of the data published in this report is prohibited. Reproduction in whole or in part or use for any public purpose is permitted only with the prior written approval of Swiss Re, and if the source reference is indicated. Courtesy copies are appreciated. Although all the information used in this report was taken from reliable sources, Swiss Re does not accept any responsibility for the accuracy or comprehensiveness of the information given or forward-looking statements made. The information provided and any opinions, projections and other forward-looking statements made are for informational purposes only and/or solely the views or opinions of the author(s), and in no way constitute or should be taken to reflect Swiss Re’s position, in particular in relation to any ongoing or future dispute. In no event shall Swiss Re be liable for any loss or damage arising in connection with the use of this information and readers are cautioned not to place undue reliance on forward-looking statements. Under no circumstances shall Swiss Re or its Group companies be liable for any financial and/or consequential loss relating to this report. Swiss Re undertakes no obligation to publicly revise or update any forward-looking statements, whether as a result of new information, future events or otherwise. This report does not constitute legal or regulatory advice and Swiss Re gives no advice and makes no investment recommendation to buy, sell or otherwise deal in securities or investments whatsoever. This document does not constitute an invitation to effect any transaction in securities or make investments.


Contributors

ETH Zürich

Prof. Dr. Giovanni Sansavini

Dr. Paolo Gabrielli

Dr. Blazhe Gjorgiev

Behnam Akbari

Linda Brodnicke

Kate Lonergan

Raphael Wu

Swiss Re

Dr. Thomas Kocher

Jan Berger

Werner Collenberg


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Changing energy mix and its impact on grid stability

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