Electricity is the most versatile and easily controlled form of energy and, for this reason, it is considered an absolute necessity in modern society. However, with respect to other commodities, electricity cannot be efficiently and economically stored in large quantities and, as a consequence, it must be consumed at the same time is it generated. The transportation of electric power from power generators to consumers is achieved via the so-called electric power systems.
A power system, depicted in Figure 4.1, is composed of three main sub-parts: the generation, the transmission, and the distribution system. The generation system comprises generation units that supply power to the system. Some examples of traditional generators are thermal (oil, coal, natural gas, etc.), hydro, or nuclear power plants. Even though significant differences exist between them both in terms of controllability and responsiveness, the most important feature of these units is that they can be considered as being deterministic, i.e., they can be operated so as to output predefined power levels. The transmission system is composed of electrical lines that transport the power from the generation side to the distribution system. As high voltage AC transmissions provide a very efficient way to carry power with reduced losses, typical voltage levels range between 130 kV and 400kV. To guarantee the proper functioning of the network, a number of mechanisms and electrical devices are deployed that maintain voltage levels and phases (e.g. voltage-regulators, step-up/- down transformers, etc.). Beside voltage and phases, also the power balance and the frequency
4.1 Introduction 45
Figure 4.1 – Overview of a traditional electrical power system. Source: [58]
must be controlled at all times. In particular, this is achieved by specialized generation units that exhibit fast reaction times. Finally, the distribution system is in charge of feeding power to the loads such as households, hospitals, commercial buildings, and small to medium sized industries, among others. It has a similar structure to the transmission network but it works at lower voltage levels and it covers a significantly smaller geographical area.
In order to understand the operation of an electrical power grid, beside its physical structure, one should also consider another important component: the electricity market. The function of electric- ity markets is to facilitate the trading of electric power products between consumers and producers. Most electricity markets worldwide are deregulated meaning that competition between different ac- tors is encouraged in order to increase the overall efficiency of the system. In Switzerland, electric energy is traded in the European integrated wholesale market, EPEX SPOT, that groups France, Germany, Britain, Switzerland, Austria, Belgium, the Netherlands, and Luxembourg. Every day, D, energy bids are collected until 12:00 of the current day for each hourly spot of the next day, D+1. Then, at market closure, following a clearing mechanism, the best bids are accepted and the final index of prices together with production and consumption schedules are communicated by the EPEX to each participant. At the beginning of the next day, D+1, these schedules come into action and should be respected by the participants. However, possible adjustments can be made, either at one hour or 15 minutes resolution and up to 45 minutes before delivery. In the central European area, such modifications take place in the form of energy trades, called intraday transactions, in the so-called intraday market.
All the previously described markets are energy markets. However, even after intraday adjustments, discrepancies between generation and consumption during real-time operation can arise and should be appropriately managed. Balancing imbalances at this faster time scale is the responsibility of Transmission System Operators (TSOs) that are typically state-owned monopolies in charge of guaranteeing the proper functioning of transmission systems. The Swiss TSO, SwissGrid, achieves this goal by procuring reserve generation capabilities, called Ancillary Services (AS), that function
Figure 4.2 – Sequential activation of frequency control reserves for a power plant outage in France. Source: www.swissgrid.ch
as a backup to cover the real-time mismatch between generation and consumption. Although different TSOs might procure different types of AS, the following categories are generally present:
• Frequency control (active power reserves) • Voltage control (reactive power reserves) • Black start
• Compensation of active power losses
The technical specifications of such services are described in great detail in [119] while the economic aspects are investigated in [120].
In this thesis the main focus is on frequency control reserves, which is the service specifically designed to maintain the system frequency at its rated value (50 Hz in Europe). The frequency is, in fact, a direct and instantaneous measure of the imbalance present in the network so that a value above 50 Hz indicates a surplus of energy and a value below 50 Hz indicates a shortage of energy. To make sure that grid contingencies are managed at various time scales, TSOs control the
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frequency in three steps which are activated in a sequential fashion (Fig. 4.2): primary, secondary, and tertiary control.
Primary Frequency Control (PFC): It represents the fastest regulating layer after a con- tingency. For instance, in Switzerland an activation requirement of 30 seconds is imposed. Also, it is completely decentralized with each provider responding to local frequency measurements in a proportional fashion according to its droop characteristic [70]. Due to the proportional control and the lack of communication, PFC is employed to solely stabilize the frequency without remov- ing steady-state frequency errors. In theory, PFC should be released after 5 minutes when the next layer is activated. For this reason, the energy requirements of PFC are quite small and, the providers of this service are compensated only for the provided capacity (and not for the energy).
Secondary Frequency Control (SFC): After the frequency is stabilized, and at most after 5 minutes, SFC is activated to restore the nominal frequency of 50 Hz. As opposed to the decentralized structure of PFC, SFC is centrally regulated by the TSO responsible for the control area where the contingency took place. For the Swiss control area, SwissGrid computes the so-called area generation control signal (AGC) which is the output of a Proportional-Integral (PI) controller with the area control error as input [64]. The signal is then broadcast every 2-4 seconds to the SFC providers which react accordingly by modulating their active power injection. If the AGC assumes a positive value, the provider should increase the power production and vice versa. In Switzerland, SFC reserves are contracted in a market setting where generators bid their capacity in weekly auctions. SwissGrid requires symmetric capacity meaning that the providers should be able to both increase and decrease their active power injection with respect to a pre-defined baseline injection.
Tertiary Frequency Control (TFC): It is the last layer to be activated in case of a major and persistent contingency. Its main purpose is to relieve SFC reserves after a period of 15 minutes. As of today, TFC is activated in a manual or semi-automatic fashion by special electronically transmitted messages sent by the TSO to large generating units that are required to adapt their power production levels.