Introduction to Frequency Control and Inertial Response Schemes for the Future Power Networks

Abstract Future power systems face several challenges: (i) the high penetration level of renewable energy from highly variable generators connected over power converters, (ii) several technologies for energy storage with very different time constants, some of them using power converters as an interface to the grid, and (iii) a pan-European transmission network facilitating the integration of large-scale renewable energy sources and the balancing and transportation of electricity based on underwater multi-terminal high voltage direct current (MTDC) transmission. All of them have an element in common, high power converters that decouple the new energy sources from the pre-existent AC power systems. During a system frequency disturbance, the generation/demand power balance is lost, the system frequency will change at a rate initially determined by the total system inertia. However, future power systems will increase the installed power capacity (MVA) but the effective system inertial response will stay the same nowadays, because the new generation units based on power converters creates a decoupling effect of the real inertia and the AC grid. The result is deeper frequency excursions of system disturbances. A considerable reduction in the ability to overcome system frequency disturbances is expected, the inertia response may be decreased. The aim of this chapter is to present the fundamental aspects of system frequency control and inertial response schemes for the future power networks.

Keywords Frequency controller ° Frequency stability ° Future power networks ° Power system ° Protection scheme ° Wind turbine generator

Introduction

Electrical power systems have been developed, over more than 50 years, to deliver electricity to the end users. This approach requires to use a vital infrastructure to link the producers of the electric energy and the consumers [1]. This approach of power systems design and operation has served their purpose with great success for many decades mainly because they were developed to meet the needs of large and predominantly carbon-based energy producers located remotely from the load centers. Nowadays, governments around the world are striving toward three key issues:

• Climate change. The climate conference in Kyoto for the first time had inter- nationally binding targets for the reduction of greenhouse gases like carbon dioxide, methane, nitrous oxide, hydro fluorocarbons, and sulfur hexafluoride until 2012 by 5.2 % compared to 1990. The United Nations Climate Change Conference was held in Cancun in 2010 [1], and it agreed only to continue the implementation of the Kyoto Conference without setting new targets for the period after 2012. However, the European Union (EU) has been seriously committed to CO2 reduction. In 2007, it agreed to the target triple of supply, competitiveness, environment, to reduce the CO2 emissions by 2020 by at least 20 % compared to 1990. Several scientific works argue that this would not be sufficient to limit the effect of warming process in reach heating of the atmosphere within 2 oC. For this reason, the EU considered to increase the reduction target to 30 % by 2020. By 2040, emissions are to be reduced by 60 %. With the use of appropriate technologies, no CO2 should be emitted any more by the power generation industry by 2050 [2]. Reducing the greenhouse gas emissions by 80 % is the specific target of the UK government by 2050 [3]. This target on the country’s emission reduction targets is defined in the Climate Change Act 2008 [4]. De-carbonizing the power sector is the key factor to reach this objective, and will enable further low-carbon choices in the transport sector (e.g., plug-in hybrid and electric vehicles) and in buildings (electric heat pumps).

• Energy security. Over the coming decades, governments around the world face the daunting challenge of meeting the energy needs of a growing and developing world population while mitigating the impacts of global climate change. Security of supply is an important goal of energy policy in many countries around the world. The importance of energy security derives from the critical role that energy plays in all aspects of everyday and business life [5]. As demand for resources rises within today’s turbulent global markets, supply chain vulnerability is becoming a significant issue. Global sourcing has created more complex and increasingly risky supply chains. Severe energy security has serious implications for social, environmental, and economic well-being. The conversion of the centralized power generation structures with the consumption of mostly imported primary energy like coal, oil, gas, and uranium to more decentralized renewable power plant systems opens the chance of reducing the import dependence from fossil energy sources. Europe as a whole is a major importer of natural gas. Although second to Norway as a supplier to Europe, Russia remains one of Europe’s most important natural gas suppliers. Europe’s natural gas consumption is projected to grow while its own domestic natural gas production continues to decline. Increasing energy efficiency is clearly the most cost-effective part of the energy revolution. Overall, in the EU the possible energy savings by energy efficiency measures and in the energy conversion process of coal, oil, gas, and uranium to electricity and heat are twice as much the energy generation potential of renewables. The EU Directive on the Energy Performance of Buildings (EPBD) [6] provides the guidelines for the reduction of 40 % of the total energy consumption in the EU. This is also true for the electricity consumption. The European Commission published in 2011 a proposal for a Directive on Energy Efficiency in order to achieve the 20 % saving goal of the EU until 2020, where a broad mix of measures are proposed. The UK government has been working on energy security for years, making sure consumers can access the energy they need at prices that are not excessively volatile. It has been reached by a combination of its liberalized energy markets, firm regulation, and extensive North Sea resources. The Department of Energy & Climate Change of UK is actively working in several aspects to guarantee that the energy system has adequate capacity and is diverse and reliable [7].

• Economic development. Development of the electric power system must con- tribute to growth and minimize costs to the consumer. A right balance between investing in generation, nongeneration balancing technologies (i.e. storage, demand-side response, and interconnection), and network assets is necessary. In addition, efficient operation of the power systems is critical to maximizing the efficient use of assets across the system. When conventional power is substituted by wind power, the avoided cost depends on the degree to which wind power substitutes each of three components—fuel cost, O&M costs, and capital. The economic competitiveness of wind power generation will depend on short-term prediction, and specific conditions for budding into short-term forward and spot markets at the power exchange. Some calculations demonstrate that although wind power might be more expensive than conventional power today, it may nevertheless take up a significant share in investors’ power plant portfolios as a hedge against volatile fossil fuel prices [8]. Continuing research and development work is needed in order to ensure wind power is to continue reducing its generation costs for sustainable economy growth.

Whilst current networks presently, fulfill their function, they will not be sufficient to meet the future challenges as described above. These challenges require technical, economic, and policy developments in order to move toward lower carbon generation technologies as well as higher efficiency devices and systems.

The radical changes that power systems are undergoing will change the land- scape of the future power networks; they face several technical challenges char- acterized by:

a. High penetration levels of renewable energy from highly variable generators connected over power converters. Changes in generation portfolio of power systems will be occurring at a time when the demand on the system is expected to increase significantly. If the environmental targets that are being imposed on the operation of power systems are to be met then large scale, more than 50 GW, renewable generation must become commonplace in the future power system. Wind generation, either onshore or offshore, is currently the primary scalable renewable generation technology that is commercially avail- able. The future targets for installing wind power in GB are: total of 26 GW (9 GW onshore and 17 GW offshore) of wind generation by 2020 and 47 GW (10 GW onshore and 37 GW offshore) of wind generation by 2030 [9]. The very fast development of fully rated converter wind turbine generator offers both enlarged capabilities and lower price per MW capacity. Increasing the penetration levels of power converter-based wind power will produce a dis- placement of traditional synchronous generation services without offering equivalent technical performance.

b. Technologies for energy storage with very different time constants, some of them use power converters as an interface to the grid. One of the greatest challenges of operating a power system is that electrical energy cannot be directly stored. Power systems must keep a balance between power generation and demand in the system in real time. This balance will become complex in the future power systems due to issues such as high variability provided by renewable energy resources. Electricity storage system (ESS) enables elec- tricity that is generated at a point of low demand to be used at a time of high demand and allows the operator to capture the difference between prices at their peak and trough. Large-scale storage (bulk) is connected to the transmission system and tends to have relatively large power output and long periods over which that power can be provided. Storage systems can also operate on a smaller scale (distributed) connected to the distribution network. Energy storage technologies have the potential to support the future system integration. Preliminary analysis in UK suggests an additional storage could be installed in the range of 1–29 GW under certain future scenarios by 2050, of which distribution storage is estimated to dominate the bulk storage, due to the savings from avoided distribution network costs [10]. There are several technologies already commercially available for distributed and bulk ESSs. However, they are technically and/or economically very different between them, and there is expectation for radically different and new technologies. Technologies will respond accordingly to provide a secure and an economical operation; they have a very different time response and interface to power system creating a complex situation in terms of operation and control.

c. A pan-European transmission network facilitating the integration of large- scale renewable energy sources and the balancing and transportation of electricity based on underwater multiterminal high voltage direct current (MTDC) transmission. Interconnection allows connected markets to import and export electricity according to the market prices on either side of the interconnector. Increased amounts of interconnection have the potential to bring savings to the system where connected markets have different generation and/or demand profiles to trade. In such circumstances, interconnection could result in generation capacity being dispatched more efficiently and reducing the total generation capacity required. The existing power grid in Europe is a highly interconnected system, spanning the whole of Continental Europe with con- nections to neighboring systems, e.g., in Scandinavia (Nordel), the UK, and Russia. The current structure of this meshed, supranational system was largely influenced by available generation technologies. The UK electricity network is connected to the systems in France (National Grid and Réseau de Transport d’Electricité, 2 GW), Northern Ireland (IFA, 2 GW), and the Netherlands (BritNed, 1 GW) through ‘‘interconnectors,’’ with others under construction or planned. Potential future interconnector opportunities include interconnectors between UK and Belgium (Nemo Link), Norway (2 GW), France, Denmark, and Iceland. HVDC has become a technology of increasing relevance to modern power systems, especially for interconnections and integration of power coming from renewable resources. There are several advantages to the use of HVDC system, but two of them are found suitable for massive deployment in the future power systems: it allows a high efficiency on the bulk power transmission over long distances and it provides a very high controllability in terms of power flows maximizing the integration of variable power coming from renewable energy resources. The use of HVDC for interconnec- tions enables to isolate electrically the neighboring systems, it allows connecting power systems that are not synchronized or do not even have the same frequency. However, this advantage comes with an issue, HVDC links offer no natural response to a frequency deviation or support for frequency control, affecting the power system security.

All technical challenges described above have an element in common; high power converters that decouple the new energy sources from the pre-existent AC power systems (see Fig. 1).

In basic works, the frequency in power systems represents the balance between generated power and demanded power. During a system frequency disturbance the generation/demand power balance is lost, and the system frequency will change at a rate initially determined by the total system inertia. However, future power systems will have a frequency response (FR) very different from actual systems. The future power system will increase the installed power capacity (MVA) but the effective system inertial response will stay the same nowadays, this is because the new generation units based on power converters creates a decoupling effect of the real inertia and the AC grid. The result is deeper frequency excursions of system disturbances. A considerable reduction in the ability to overcome system frequency’s disturbances is expected, the inertia response may be decreased. The inertial response of the system might be negatively affected with devastating consequences for system security and reliability.

Large Scale Renewable Power Generation-0168

The objective of this chapter is to present the fundamental aspects of system frequency control and inertia response schemes for the future power networks. This chapter is developed not to be intended as a generic case of all possible future scenarios and technologies, instead it focuses on the expected development to the UK system. The organization of the chapter is as follows. Section 2 presents the main concepts of system frequency response (SFR) in the classical power system. Section 3 presents the controller used to enable FR on wind power, details of wind turbine, wind farm, and power system level are presented. Commercial experi- ences on synthetic inertia and inertia requirements in several countries are pre- sented in Sect. 4. Section 5 presents a general overview of the main control strategies used in order to enable FR of offshore wind power connected using multiterminal HVDC systems. Section 6 closes the chapter with the main con- clusions and perspectives of future work.

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