Adjusting supply and demand for an electrical balance

Adjusting electrical supply and demand

Article by Ayoub El Bouhali, studying at mS. Optimization of energy systems at the Paris School of Mines. To view the school's review, Click here.  

A multi-term exercise:

Electricity is a convenience that is not stored on a large scale. Thus, the supply-demand balance is to be satisfied at alltimes. It is RTE, the manager of the transport network, which is in charge under Article L. 321-10 [1] of the energy code of this mission with all the time scales it involves. Indeed, the mastery of the supply-demand balance, for a D-Day, begins with theforecasting exercise that RTE to anticipate well in advance investments (capabilities, interconnections, etc.) to meet demand on the same day. This forecast year is updated regularly until the day before the delivery day.

In parallel to RTE's activity, market participants are encouraged,through a system of spread-by penalties, to balance their contractual is equipped in advance (injection or drawing responsibilities). In this context, they are named in Article L. 321-15 [1] of the energy code "balance-responsible." To fulfill their mission, balance managers source themselves at different maturities on the electricity markets. Three years before the delivery day, they can already trade in futures markets (derivative markets) various physical and financial products (futures, forwards, swaps, etc.), and balance their portfolio closely. On the day of delivery, balance managers are able to balance themselves in the sub-daily market up to five minutes before delivery [2].

Beyond that, the network manager has complete control over supply and demand balance. On this beach it appeals in case of imbalances between supply and demand three types of reserves: Primary, secondary And tertiary era ; to restore the frequency to its face value of 50 Hz.

The different deadlines involved in electrical balance

Frequency adjustment and supply-demand balance, what relationship?

The electric transport network is mesh at Europeanlevel. For its proper operation, there is a common obligation for all production groups in the European plate to run their alternators at the same speed. This is called synchronization of alternators.

Frequency is nothing more than the number of cycles that the electrical wavelength makes. For an alternator, it corresponds to the number of turns per second of the rotor multiplied by the number of electromagnets in the stator, which will be noted. Thus, the frequency is expressed according to the speed of rotation of the tree (rad/S) as follows:

Alternate frequency formula - electric balance

The result is this equation (1) and the synchronicity hypothesis, which the electric wavelength has the same frequency throughout the Europeanplate.

In its operation, the alternator receives mechanical power (W) which creates an engine torque on the tree (N.m). The relative movement of the rotor in relation to the stator induces an alternating current in the coil of the stator and thus an electrical force (Laplace force) which in turn applies an electric torque opposing the engine torque.

In the case of no losses (simplifying hypothesis) the principle, preservation of the kinetic moment applied to the rotor, in addition to the relationship (1) give the following equation:

formula for electric balance

Where (kg m2 s'1) is the moment of inertia of the rotor and (W) electrical power. According to (2), to balance when the injected mechanical power (production) equalizes the electrical power extracted (consumption) the acceleration of the engine shaft is zero and the frequency is stable. On the other hand, when consumption is higher than production, the alternator tree slows down and the frequency of the electrical wavelength decreases and vice versa if production is higher than consumption.

Note that according to (1) and (2) and the assumption of synchronization of alternators, the network manager can simply monitor and frequency to track in real time the supply-demand balance.

What happens if the frequency deviates?

We saw in the previous section that frequency control is intimately linked to the balance between production andconsumption. gold the latter fluctuates continuously ; production cannot follow it instantly, to avoid variations in frequency. So there is a range in which these variations are eligible. In metropolitan France, this range is 50 Hz - 0.5Hz. From 49 Hz, automatic discharges of consumption occur;  and beyond a few Hz production groups separate from the network to avoid being damaged [3]. It is a cascading phenomenon that quickly leads to the collapse of the frequency.

Fortunately, this phenomenon remains very rare, as the network manager reacts in real time to the deviation of frequency through systemservices. The latter are composed of two reserves, the primary and secondary reserves, which operate at different time and power scales.

The primary reserve (Frequency Containment Reserve) is automatically activated (down or up) in 15 to 30 seconds to maintain the frequency within its allowable range. It is sized at European level at 3000MW, to compensate for the simultaneous loss of the two largest production groups present on the European plate. France contributes 540 MW [4] . Since 1 January 2017, the primary reserve has been contracted by weekly tenders, jointly organised by RTE and its German, Austrian, Belgian, Dutch and Swiss counterparts.

The activation of the primary reserve restores frequency stability, but is generally not sufficient to reach the targeted 50 Hz. Thus, the country's automatic Frequency Resortion Reserve automatically activates (up or down) in less than 15 minutes to restore the frequency to its facevalue. Unlike the primary reserve, the secondary reserve is therefore national and is sized in France between 500 MW and 1180 MW, depending on the time slot and the time of year. All producers operating production groups of more than 120 MW are obligedto allocate part of their power to it.

The tertiary reserve and the adjustment mechanism

To maintain the balance between supply and demand, the network manager uses another power reserve that is not referred to as a "system service," the Manual Frequency Restoration Reserve. Unlike system services, this reserve is manually activated by a dispatcher to supplement the secondary reserve if it is insufficient.

The tertiary reserve is formed through the adjustmentmechanism. Introduced since 2003 [5], this mechanism requires all producers connected to the transport network to provide RTE with their power not technically used (power not used but which is technically mobile) on the market. adjustment: a single-buyer market (TEN) that takes place 2 hours before delivery and in which consumers, producers connected to the distribution network and foreign players can voluntarily participate. In the two previous cases, it is referred to as an uncontracted tertiary reserve. Understood, there is a part of the so-called contracted tertiary reserve.

Indeed, to ensure the availability of sufficient reserves in the adjustment market, RTE contracts annually through tenders with producers (or erasers)to provide it with capabilities on the market. [6], [7]. Depending on the response time of these capacities, there are two types of reserves: fast reserves, which can be mobilized in less than 13 minutes, and complementary reserves that can be mobilized in less than 30 minutes. The winners of the tender are paid for their abilities over the entire duration of the contract (fixed premium) and their energy paid only in the case of activation (variable premium).

When activating an adjustment offer (contractualized or not), the network manager is required under Article L.321-10 of the Energy Code [1] to take into account "the economic order of precedence between the adjustment proposals submissive." The energy of the activated adjustment offers is settled on the basis of the price of the offer formulated by the adjustmentactor.

article on electrical balance - blog NextFlex

The kinetic reserve and the role of inertia

Electricity is mainly produced by technologies, which involve alternators at the end of the production line. The latter constitute rotating masses, sometimes tens of tons, and therefore a very important deposit of inertia for the electrical system.

Note that according to the equation (2) the temporal variation in frequency is all the slower as the inertia is large. We are talking about a stabilizing effect of inertia, which naturally opposes the slowing or acceleration of trees in the means of production and thus deviations in frequency, which limits the need for balancing.

Moreover, the rotating masses of the means of production act physically as steering wheels of inertia. They store the rotational kinetic energy (proportionate to the inertia and square of the rotational speed) and immediately return it to the grid, by electromagnetic coupling, in the event of a transient drop in frequency. This is called a kinetic reserve.

The kinetic reserve is essential for the instantaneous stability of thenetwork, if it is not sufficient, the electrical system may collapse before the other reserves can be activated [8]. A major shift to means of production without rotating masses, such as photovoltaic solar, must be accompanied by technical, regulatory and commercial devices to preserve the stability of the electrical system.

change in frequency following 2800 MW production incident - electrical balance
bibliography
[1] "Energy Code Legifrance." [Online]. Available on: https://www.legifrance.gouv.fr/affichCode.do?cidTexte=LEGITEXT000023983208. [Consulted: 15-May-2019].

[2] "EPEX SPOT SE: France." [Online]. Available on: https://www.epexspot.com/en/product-info/intradaycontinuous/france. [Consulted: 01-May-2019].

[3] RTE, "How to offloading between RTE and Distributors," 2008.

[4] "System services and adjustment mechanism." [Online]. Available on: https://www.cre.fr/Electricite/Reseaux-d-electricite/Services-systeme-et-mecanisme-d-ajustement. [Consulted: 15-May-2019].

[5] "Adjustment Market," RTEFrance, 15-Sept-2014. [Online]. Available on: https://www.rte-france.com/fr/article/marche-d-ajustement. [Consulted: 15-May-2019].

[6] RTE, "Charge Book," 2019.

[7] CRE, Deliberation of the Energy Regulatory Commission of 21 June 2018 approving the terms of the 2019 tender for rapid and complementary reserves. 2018.

[8] Mr. Drouineau, "Forward-looking modeling and spatial-temporal analysis: integration of electrical grid dynamics," 2011.

[9] "Implement all levers to ensure balance," RTE France, 15-sept-2014. [Online]. Available on: https://www.rte-france.com/fr/article/mettre-en-oeuvre-tous-les-leviers-pour-assurer-l-equilibre. [Consulted: 15-May-2019].
f-p-/2 - electric balance blog article NextFlex

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