A Survey on Inertia Related Challenges and Mitigation Measures Erik Ørum Mikko Kuivaniemi, Liisa Haarla, Anders Jerkø, Inge Stenkløv Minna Laasonen Energinet Fingrid Oyj Statnett Fredericia, Denmark Helsinki, Finland Oslo, Norway Fredrik Wik, Robert Eriksson, Niklas Modig, Pieter Schavemaker Katherine Elkington Svenska kraftnät E-Bridge Consulting B.V. Sundbyberg, Sweden Oosterbeek, the Netherlands Abstract—The amount of kinetic energy in the Nordic power European system, due to the commissioning of more system decreases due to structural changes in the power interconnectors. system. In this paper, the Nordic Transmission System Operators (TSOs) provide a summary of a survey, which was More coupled markets, including exchange of sent out to some small- and medium-sized synchronous areas, balancing reserves. in order to learn from their experiences with low kinetic Changed load characteristics, for example industrial energy situations. The paper summarizes the responses received, thereby providing a view on: the current and future loads are connected to the grid through power- assessment of inertia in the system (including key drivers electronic converters. behind the change), and mitigation methods for low inertia Frequency stability of power systems is highly situations. dependent on the amount of kinetic energy in the system, as demonstrated in Fig. 1 [2]. The figure shows the response in survey; inertia; frequency stability; renewable energy; system frequency during a sudden loss of production. synchronous system I. INTRODUCTION The Nordic power system is going through large structural changes which challenge the way the Transmission System Operators (TSOs) plan and operate the system. The changes will pave the way to the next generation power system, which will secure the future welfare, value creation, and help us reach carbon-neutrality. The main changes are [1]: The share of renewable energy sources in the energy mix is increasing, and more energy will to a larger extend be produced by small-scale, renewable and distributed power plants. Large-scale wind farms are also foreseen. Most of those units are connected by means of power-electronic converters. Nuclear power plants are de-commissioned earlier than initially planned in Sweden, while Finland is Figure 1 The effect of the amount of kinetic energy on the behaviour of constructing new. frequency after a loss of production with (solid) and without (dotted) Frequency Containment Reserves (FCR) Denmark and Finland have shut down many fossil- fueled power plants. Reduced kinetic energy has an impact on the ability of the system to resist changes in frequency due to imbalance The Nordic power system will be more strongly between power production and consumption. All rotating connected through HVDC interconnectors with machines connected synchronously to the system contribute other synchronous systems, for example the central to this kinetic energy. Much renewable generation Synchronous Country Min. / Min. / Specific connected through converters to the grid continues to area or Max. Max. replace the conventional generation. As a result, the amount company load in kinetic name GW energy of kinetic energy in the system decreases and may reach GWs critical levels unless no countermeasures are taken. Such connected with an situation may result in involuntary production or HVDC link. consumption shedding after the occurrence of a Transpower New 1.3 / 11 / 25 dimensioning incident. Furthermore, an increase in HVDC Zealand 2.2 import capacity may also contribute to the decrease in the (South Island) kinetic energy of the system. Faroe island Denmark 0.02 / Not The structural changes identified in the Nordic power 0.05 available AEMO Queensla 14 / 30 72 / 50 Tasmania has a system are not unique, and similar changes are occurring in (Australian nd, HVDC connection; other systems. Small and medium-sized synchronous Energy Victoria, the other states systems are likely to already have experience and Market New belong to one knowledge on how to handle the challenges. To benefit from Operator) South synchronous the experience gained, a survey was sent out by the Nordic Wales, system. and TSOs in the autumn of 2016. Tasmania in II. SYNCHRONOUS SYSTEMS IN THE SURVEY Australia ESKOM South 19 / 35 Not In total 10 answers were received, Table I shows an Africa estimated overview of the systems which answered the survey. Rhodes An island 0.032 / 0.15 / Moreover, the corresponding values for the Nordic in Greece 0.191 0.63 synchronous system are also shown. Hydro- Canada 15 / 39 60 / 160 Synchronous area is Quebec the Quebec Most systems have a market-driven dispatch, 3 systems TransEnergie interconnection. (ESKOM, Faroe Islands, and Hydro-Quebec TransEnergie) Almost all of the interconnection is have a centralized dispatch, whereas Rhodes in Greece is in located on the a transition towards a market-driven dispatch. territory of Québec. TABLE I. BASIC INFORMATION OF THE SYNCHRONOUS SYSTEMS, WHICH ARE IN THE SURVEY (THE INFORMATION IN III. THE DIFFERENT SYNCHRONOUS AREAS COMPARED THE TABLE IS BASED ON 2015 DATA) Next to the technical characteristics of the synchronous Synchronous Country Min. / Min. / Specific area or Max. Max. area, as listed in Table I, other elements do play a role when company load in kinetic assessing the inertia, and its impact, in a synchronous area. name GW energy GWs The dimensioning incident is the single event causing The Nordic Norway, 25 / 70 125 / 240 One synchronous the highest instantaneously-occurring active power power system Eastern area with four imbalance in both positive and negative direction in the Denmark transmission system synchronous system. This can for example be a trip of a , Sweden, operators (TSOs) nuclear power station, leading to a frequency dip (as shown Finland in Fig. 1), or a trip of an HVDC link exporting power to ERCOT USA 24 / 70 152 / 389 The Electric (The Electric Reliability Council another synchronous area, leading to a frequency rise. The Reliability of Texas (ERCOT) dimensioning incident, together with the inertia and Council of is the electricity frequency reserves in the system at the time of the Texas) grid and market disturbance, set the minimum and maximum frequency that operator for the the system can face under N‒1 conditions. The majority of the state of Texas. dimensioning incidents for the systems in the survey are National Grid Scotland, 17 / 53 130 / NA NG is the National depicted in Fig. 2. (NG) Wales, [3] Electricity England Transmission Production behind power-electronic interfaces does not System Operator contribute to the inertia in the system. The same applies for (NETSO), for Great HVDC links connected between two synchronous areas. Fig. Britain 3 presents the total installed generation capacities in the Eirgrid Ireland 2.3 / 20 / 46 The synchronous systems, and the installed capacity of the generation not- 6.4 power system of the contributing to inertia including HVDC capacity. Fig. 4 island of Ireland comprises of both shows the percentage values of generation that does not the EirGrid- contribute to inertia and import HVDC capacities as a share controlled Irish of the installed generation. system and the SONI-controlled Fig. 4 shows that the HVDC import together with the Northern Irish share of generation that does not contribute to the inertia system. exceeds 30 % in two systems: the UK and Ireland. For two Transpower New 1.7 / 20 / 41 Two synchronous systems, the share is 20–25 %. This does not directly Zealand 4.5 areas: North Island (North and South Island. express the number of hours when the inertia is low, but it is Island) The islands are an indicator that there may be hours when the inertia is low. Q6 Dimensioning Incidents (MW) 3000 2500 2000 1500 MW 1000 500 0 Under-frequency side Over-frequency side Figure 2 The largestTotal loss installed (dimensioning capacity incident), (GW) that can and happen the in sumthe system. of the NI =generation North Island, SI not = South Island. For Rhodes the values are 26 MW for under-frequency and 33 MW for over-frequency contributing to inertia and HVDC import (GW) 100 90 80 70 60 50 GW 40 30 20 10 0 Total installed capacity (GW) Generation not contributing to inertia + HVDC capacity (GW) Figure 3 The total installed capacity and the amount of non-synchronous generation, both in gigawatts. For Ercot the number representsThe summer share peak of deman converterd where installed-connected wind and solar generation generation capacity and areHVDC discounted based on capacity contribution and summer ratings for thermal generation are used import of the installed capacity (%) 45,0 40,0 35,0 30,0 25,0 20,0 Percentage 15,0 10,0 5,0 0,0 Generation not-contributing to inertia (%) HVDC capacity (%) as a share of the installed capacity Figure 4 The share of the (combined) installed capacity of generation not contributing to inertia and the transmission capacity of HVDC import in percentages of the system installed capacity. The TSOs that consider low inertia to be an issue are marked with a grey box. The TSOs that consider inertia to be an issue are marked secure operation. ERCOT, National Grid (NG), and New with a grey box in Fig. 4. Fig. 4 also shows that Eirgrid and Zealand Transpower dynamically dimension the needed ERCOT have a quite high share of generation and HVDC disturbance reserves according to the system state. ERCOT import not contributing to the inertia in the system, but do determines the minimum requirement of Responsive not consider low inertia to be an issue today. ERCOT states Reserve Service (RRS) based on the expected system inertia that they have enough inertia at the moment, and that they conditions. RRS is a service used for frequency response are monitoring the situation. In 2014, ERCOT performed and in case of energy scarcity. Load resources tripping with dynamic simulations for dimensioning their reserves. Eirgrid a 0.5 s delay is a part of this service.