Report back from Cigré Paris session 2018

Cigré is a global community for the collaborative development and sharing of power system expertise. Established in Paris, France in 1921, Cigré has a long history as a key player in the development of power systems. An organisation connecting power system professionals from all over the world, Cigré runs a worldwide programme comprising high value events by its study committees (SCs), national committees (NCs) and other groups, spanning large events such as symposia and colloquia as well as numerous local conferences. The organization produces reference publications based on practical experience and analysed data. The programme culminates every two years in the Paris Session, the world’s number one global power system event, unique for its thought leadership.

Fig 1: Groups of interest in the Cigré structure.

Cigré operates in16 domains of work, each with its dedicated study committee. The organization has four current strategic directions:

  • Future power systems: focus on the future grid
  • Best use of existing systems
  • Environment and sustainability
  • Unbiased information for all stakeholders

Within the strategic directions, current pressing challenges are:

  • Renewable energy sources
  • Intermittency of renewable power generation
  • Growing environmental requirements
  • Limitations to build new transmission infrastructures
  • Architecture of networks and systems
  • Maintaining the existing power systems
  • Transmission of large amounts of power over long distances
  • Cyber security

A report back on the Paris session was held on 18 October, at which summaries of the SC meetings were presented by members who attended the session. Two major topics emerged which appeared in almost all summary reports, namely renewable energy/distributed generation, and energy storage. Other important topics to emerge were the optimisation of systems and solutions, digitisation, substation design and optimisation; as well as HVDC/MVDC networks.

Renewable energy and distributed generation

Distributed generation (DG) continues to grow organically in installed capacity, and in some parts of the developed world is beginning to exceed peak demand capacity. Five wind turbines (two per hour in China alone) and 30 000 solar panels are installed globally every hour. It is expected that 7600 TWh of renewable generation will be installed by 2021 (28% of total global power generation from a total of 27 000 TWh).To keep the system stable, and make the most of “zero marginal cost” generation, network operators need to consider interconnection (at smart-grid level, but also at national, international and even intercontinental level), buffering (storage) and cooperation (market facilitation, sector coupling, community energy, etc.)

Most notable of the influences has been the focus on greater environmental sustainability, which drives the commitment to increase renewable generation globally. There is a shift away from a conventional generation dominated portfolio to diverse portfolios which now includes greater levels of distributed generation, renewable generation and demand participation. The increase in alternative generation sources presents a range of operational challenges for the power system.

The challenges are to continue to ensure power system security, stability and reliability at high levels of renewable energy resources. Power system inertia, frequency and voltage control, system stability all have to be carefully considered in terms of technical and commercial requirements. Managing frequency control and increasing ROCOF levels and methods of quantifying contributions of inertia, fast frequency response and governor response required to maintain sufficient stability need attention. The key to this is having adequate tools to monitor current and future system operating conditions, including static and dynamic security assessment (SDSA), phasor measurement units (PMUs) and wide area monitoring/control (WAM/WAC).

In South Africa, an increase in renewable energy can easily go beyond what is planned in the IRP if global exponential growth is used as benchmark, especially small-scale PV. Sustaining grid stability with the presence of large-scale PV and wind will require that plans must keep large turbo and hydro generators in service for inertia and base load support, invest in large-scale battery storage, and consider converting decommissioned generators into synchronous condensers.

Increased renewable resources result in a highly volatile grid with associated significant load swings. Presently, coal-fired units are used to smooth out grid unbalances, but these large machines are not designed for constant load cycling, and are becoming more unreliable, which will require alternative solutions to cover load swings A high percentage of solar requires generation support with very high ramping needs as well as instantaneous availability due to the uncertainty of the energy source. Grids are volatile and need large quantities of fast response flexible support. Existing legacy fleet generation is not designed for this highly flexible support and is operated beyond its intend limits with devastating consequences.

Grid codes have been adapted to force component suppliers and component owners to comply with the requirements of this highly flexible grid, but have not taken component limitations and cost into account. Investment and planning of renewable energy sources must also cater for the investment in new highly flexible generation with storage capability, also maintain reliable continuation of existing legacy generation to sustain inertia and stable energy supply in an attempt to maintain grid stability.

Increasing amounts of renewable energy generation and power electronics devices are causing a reduction of system inertia. Based on new approaches to system inertia calculations, and the increased need for it, a suitable type of new AS (inertia provision service) was proposed. Higher non-spinning generation also calls for monitoring, management and synthetic supplementation of inertia. Interruptible loads or load curtailment deals with customer price trade-offs are being considered globally as a means to manage supply and demand balance. Similarly, adaptive generating agreements as an alternative to conventional connection schemes have been proposed.

System inertia is eroded by increasing amounts of renewable energy generation and power electronics (PE) devices. The impact of system inertia reduction on the rate of change of frequency (ROCOF), which can reach a high value greater than 6 Hz/s when a system disturbance occurs which involves either a loss of generation, or loss of power export such as a large interconnector, is a source of concern. ROCOF events produce current fluctuations at low frequencies, at around 1 Hz. At these frequencies, the induced currents in the rotor will reach areas far inside the rotor, i.e., the rotor slot bottom. For ROCOF faster than 1 Hz/s studies need to be repeated to ensure that no damage to any rotor component will occur. Asset usage and longevity can be affected by highly variable/non-schedulable generation.

The challenges related to managing frequency control and increasing ROCOF levels are present in many countries. Methods of quantifying contributions of inertia, fast frequency response and governor response required to maintain sufficient stability are being developed. The Australian system is experiencing a decline of synchronous to asynchronous generation, causing higher ROCOF. The 2016 blackout values were 6 Hz/s while the automatic under frequency load shed (AUFLS) is designed for 3 Hz/s and had no time to operate. In future, a scheme is being designed to monitor the health of the system, the first stage uses pseudo loss of synchronism relays to send load shedding signals. The second stage is considered using PMU’s to shed load. Fast power injection by battery energy storage is also used. PV technology has been used to provide inverter-based voltage control especially in situations of light system loading (e.g. Brazil project Midas).

The lack of inertia, and intrinsic inability to manage reactive power, associated with the new alternatives of generating electricity, is a demanding reality with which the transmission system operators are learning to deal. The consequences are to put more of a burden on the conventional generating units. Generator cost could increase by 15 to 20%, if new codes are to be met, and additional construction cost to host larger generator size and weight and auxiliary equipment for the same rated power output with no additional benefit could result.

The cost of the generating unit (generator plus transformer) that fulfils the new EU grid code can be between 15 and 20% higher than the cost of the same power rating with original generating unit. Purchasing new generators and the refurbishment of old generators will require a tripartite discussion between manufacturer, customer and grid operator as the present South African grid code requirement can result in a very expensive overdesign which might not be required for a specific region. Changes to the grid due to an exponential increase in renewable energy will impact on the ability of existing equipment to maintain grid stability as existing generating equipment has design limitations which cannot be exceeded.

Utilities need to be agile and evolve fast to cultivate market systems which favour flexibility and availability, support multi-directional power flow and facilitate aggregators, prosumers and newly emerging business models; but also to attract young, skilled professionals who will not be tolerant of old-fashioned utility-style employment.

The impact of distributed resources such as embedded generation and energy storage on substation design and operation and under study, including what changes at the substation level are necessary to accommodate these into the power network and How digitalisation can help achieve these goals. Without distributed generation, the power flow was only one directional and it is easy to trip the correct breakers, but with distributed generation the power flow is in more than one direction and care must be taken not to trip interconnectors.

HVDC grids offer great opportunities for expansion of infrastructure such as integration into existing AC grids or remote renewable energy systems (RES) but require experience with fundamental components of HVDC and standards, procedures, and tools (special modelling and analysis). HVDC should form an integral part of transmission network integration options, particularly for regional interconnections and evacuation of power from RES to load centres. The integration of large amount of variable RES has an impact on technical design requirements of the HVDC transmission schemes, mostly on LCC technology as it is dependent on AC system characteristics i.e. SCR, transient and dynamic voltage stability of a network.

Tools, techniques, and data used in transmission system planning and investment decisions are needed to evaluate and enable high levels of renewables, storage, and customer flexibility at all voltage levels, especially in holistic approaches that combine technical assessments, incentives and reliability impacts on customers. Evolution of the existing and new expansion planning tools to include the value of distributed generation and customer flexibility is necessary.

Aggregated modelling of the downstream MV and LV network response continues to be a challenging task and now needs to consider DER in addition to traditional load components. It was reported from Australia that work is underway to better identify the key characteristics of DER that need to be included in future composite models.

The increase of renewables requires improved harmonic emission analysis techniques. Due to a high investment cost of compensation devices for harmonics and voltage variations caused by wind farms and other large non-linear installations, improved analysis procedures to assess voltage distortions are required. High frequency harmonics in the range from 2 to 150 kHz (due to inverters) are becoming a topical issue, and additional research activities on measurement requirements are being considered. Practical cases of equipment malfunctions are reported especially from LV networks. The 2 to 150 kHz range must be researched. The increase in fast switching power electronics will generate distortion at these high frequencies impacting the operation other electronic devices on the network.

The integration of MV/LV distributed generation requires an industry-based value chain approach for successful implementation. The initial acceptance rates are very high, and the industry must cater for all the elements involved in such a programme, including metering, installation control and utility management services. It is also essential that the QOS impact of any distributed generation installation is adequately monitored and assessed, with specific emphasis being placed on adequate invertor quality outputs.

The management of DES is most effective when aggregated into distinct groupings. Virtual power plants and microgrids are grouped into distinct manageable clusters and linked to the larger DMS system for operational purposes. Status monitoring, load forecasting capability and active and reactive power flows are improved as a result.

Microgrids and DERs introduce new ICT challenges to utilities as they are distributed with the end user infrastructure not necessarily dictated by the utility.  South Africa needs to pay attention to the ICT infrastructure requirements for microgrids and DERs as these solutions are anticipated to grow in South Africa.

All markets are grappling with how to best design electricity markets for increased RES Competitive wholesale electricity markets are nearly ubiquitous, but it takes time for transformation and appropriate market design, e.g., capacity markets, day-ahead markets, intra-day markets, network tariff principles to develop. Methods for valuation of new distributed resources and regulatory frameworks are required to enable these (e.g. storage, DSR).

Regional regulatory frameworks and competitive market designs becoming more prevalent with good experiences already in existence (USA, EU, Latin America). This requires an interface between TSOs (Eskom) and DSOs (municipalities) and energy services that could be provided from various resources and enabling regulatory rules. Guidelines to better establish and operate ancillary services markets as RES increases are required. Special zones for renewable energy development, i.e., REDZ could use special tariffs. New or upgraded network codes and market rules are being established with good lessons learnt based on extensive consultation and experience.

To prevent sudden collapse of the South African network, the following aspects have to be taken into account for future planning and investment:

  • Changes to the grid due to an exponential increase in renewable energy will impact on the ability of existing equipment to maintain grid stability as existing generating equipment has design limitations which cannot be exceeded.
  • Renewable energy is growing at a significant pace and can become uncontrollable. This will introduce network failures, as has already happened in South Australia and various other countries.
  • South Australia is now introducing battery storage at an enormous scale and very high cost. We need to be prepared, else we will end up with another asset bubble. Recovering from it will not be quick and easy as our legacy coal fired plants will be damaged beyond economical repair due to the abuse they endure to establish a renewable grid. Consequences to economies can be devastating.

Battery energy storage

Battery energy storage systems (BESS) are considered as a technically and economically viable technology. It is expected that there will be many new BESS applications in the network in the coming years. Additional research activities are required to explore different network functionalities of such systems. BESS is being deployed by many utilities and countries. It provides benefits for the deferment of MV and LV grid strengthening, and other ancillary services such as voltage and frequency control. BESS technology must however be carefully managed via technical rules and standards, to allow for coherent load forecasting and production management via the DSO and TSOs.

The benefits of multiple forms of energy storage were highlighted. This included newer forms of energy storage such as Li-ion batteries and phase change materials, but also discussed the benefits which traditional pumped hydro generation can have for a power system with high renewables. The regulatory barriers which need to be overcome in order to deploy large levels of storage were raised as a concern.

BESS has a large output capacity and multiple vendors connect various controllers, storage is not only for supplying power but also to manage and control voltage and frequency fluctuations. In each islanded system the controller must control voltage and frequency at high speed or it will not be sustainable. They use FL-Net as best high-speed communications network.

The main concerns are to clearly identify what is the purpose of the storage system, the automatic control systems are very complicated, and to ensure optimal lifespan of the batteries these need to be optimised. The storage system must achieve the network requirements and to provide enough fault level for protection functions it is often necessary to oversize the batteries. Better active power and frequency control is possible using a BESS. The greater the system disturbance, the greater the reduction of frequency deviation by using BESS. For 200 MW disturbance a 40 MW BESS can improve frequency deviation and for 50 MW disturbance a 10 MW BESS will be enough. BESS can also be used at peak demand periods to support generation. A 5 MW/0,5hr BESS was found to be enough for a 40 MW diesel generator and a 10 MW PV system.

Most unbundled DSOs are not presently allowed to connect utility-scale BESS onto the distribution network. Most large-scale experience may therefore come from vertically integrated utilities. Methods to carry out cost-benefit analyses for BESS based on service provision are still not readily available in the industry, but suggestions are being made, using tools such as multi-objective optimisation. Presently, the business case for BESS still relies on crediting multiple stacked advantages. The hosting capacity of the electricity grid requires a proper understanding, as the connection of many distributed generators may cause poor QOS, generation/load unbalances, voltage related problems and possible congestion of the grid. Advanced grid monitoring and control capabilities need to be in place to manage this. This will also allow such grids to be used for black start purposes, where BESS is present. Additionally, information sharing with respect to hosting capacities needs to be freely available.

Aggregation of behind-the-meter batteries (including EVs, residential batteries and commercial batteries) has been proposed as a means of incentivised grid service. Second-use EV batteries have been identified as a potential future source for grid energy storage. Over 500 000 t of discarded Li-ion batteries are expected by 2020. The global BESS industry is still very young, and South Africa’s intentions to install large systems will be internationally ground-breaking. Project learnings should therefore be documented carefully and published for international benefit.

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