Enable Ancillary Services bY Renewable Energy Sources (EASY-RES Project)

By C. S. Demoulias, K.-N. D. Malamaki, G. C. Kryonidis, M. Cvetkovic, U. Mushtaq, S. I. Gkavanoudis, K. O. Oureilidis, J. M. Mauricio, and E. O. Kontis

The increasing penetration of Distributed Renewable Energy Sources (DRES) has brought into focus a series of technical problems that jeopardize the stable and reliable operation of electrical power systems. EASY-RES is an EU Horizon 2020 research project aiming to the improvement of the energy security by making DRES provide a number of significant new Ancillary Services (AS) allowing, thus, high DRES penetration -up to 100%- in the grid with simultaneous decommission of large Synchronous Generators (SGs) driven by fossil-fuels.

Gaps, Opportunities, and Drivers

The stability and security of the traditional electrical power systems is largely based on the inherent properties of SGs: (i) inertia due to their rotating masses; (ii) power oscillations damping due to rotor’s damper coils; (iii) grid-forming capability; (iv) reactive power capability; (v) provision of large currents, when suddenly short-circuited. These properties formed the basis for the design of the current grid network topologies and fault-protection systems.

The growing penetration of converter-interfaced, inertia-less DRES tends to gradually replace dispatchable SGs. This, in addition to the intermittent nature of DRES (e.g. solar, wind) and their dispersion across the distribution grid, leads to increased power volatility and bi-directional power flows. The latter in turn have caused several problems: large frequency deviations, voltage regulation issues, power quality problems and fault detection/clearing issues.

Currently, typical measures to avert these issues include: increase of SG spinning reserves, grid reinforcement at various voltage levels and increased use of central electric Energy Storage Systems (ESS). However, due to their centralized approach and high cost, these actions can be undertaken only by Transmission System Operators (TSOs) and Distribution System Operators (DSOs), leaving the grid-supporting potential of DRES untapped. Yet, it is questionable if the capital and operational costs of these methods can be allocated in a fair and transparent way to the electricity stakeholders.

For all these reasons, DRES must urgently adopt a new more active role in the electricity networks. A similar active role could also be adopted by microgrids (μGs) at LV distribution grids. New AS can be provided to the DSOs and TSOs by the individual DRES and μGs in a coordinated control scheme.

Technological Innovation/Potential

The term “ancillary service” is frequently used in the technical literature to describe various functionalities of the DRES, e.g. reactive power for voltage regulation purposes, virtual inertia, primary frequency control (PFC), harmonic mitigation or even power smoothing. However, there is no established method for the simultaneous provision of these AS in a unified way by a common hardware and control algorithm. Moreover, there is still no agreed method to quantify and measure these AS. The latter is important to enable AS that are offered at the distribution system to participate in future AS markets.

In EASY-RES the following bottom-up approach is adopted:

(i) Every DRES, especially the converter-interfaced ones, will be able to offer several AS: virtual inertia, PFC, reactive power for voltage regulation, controllable contribution of currents during faults, mitigation of voltage harmonic pollution and active power smoothing. Properly and reasonably sized Fast Storage Systems (FSS), e.g. ultracapacitors, together with new control algorithms will be employed for this reason.

(ii) Methods to quantify and measure the AS offered by each DRES will be developed. From the hardware point of view, only the FSS bank and the associated DC/DC converter will be additionally needed, while the main DRES DC/AC converter may remain at its current topology, apart from a slight oversizing. On the other hand, the control algorithm is quite innovative addressing simultaneously the main DRES converter and the FSS bank, so that the DRES is seen from the grid as a virtual SG with new enhanced functionalities, that are fully controllable and quantifiable, contrary to conventional SGs.

(iii) At distribution system level, one or more Individual Control Areas (ICAs) are foreseen, where the DRES with the new functionalities are optimally orchestrated by the DSO via suitably designed ICT infrastructure for: voltage regulation, protection coordination, optimal allocation of virtual inertia and PFC and coordinated harmonic mitigation inside the ICA. In case of ICT failure or emergencies, the coordination will continue through decentralized algorithms, but not in an optimal way.

(iv) Based on the ICA topology, the type and location of the DRES, the forecasted RES and load magnitude, and the controllability of the DRES, the DSO will be able to offer quantified inertial and PFC to the upstream TSO at the ICA Point of Common Coupling together with the associated cost of these AS. In other words, the ICA will be seen by the TSO as active virtual power plant with predictable frequency dynamics. The proper definition and quantification of every AS will enable the per-unit estimation of the investment and operational cost for the provision of each AS, either individually or in aggregated form, leading in this way, to a proper remuneration within the frame of existing or new AS markets at the ICA level.

Implementation

In the EASY-RES approach, the DSO is able to control each individual DRES and provide AS in an aggregated form either within the ICA or to the upstream TSO. At the ICA level, the provision of the AS by DRES and ESS are optimally coordinated by Central Controllers at LV and MV level. These coordinated algorithms obey technical constraints posed by the DSO (e.g. thermal limits, overvoltage, total harmonic distortion, power and energy losses, etc.) considering the DRES type and location inside the ICA.

Since the DRES will offer controllable inertia and PFC, a methodology is introduced for the evaluation -at given future time interval – of the amount/cost of inertial response and PFC offered by the DSO to the upstream TSO. The cost is also a variable, because it depends on the aforementioned technical constraints and parameters.

Another tool will enable the DSO to develop the equivalent dynamic model of each ICA and pass it to the TSO for enhanced stability analysis.

To enable the proper remuneration of the AS, EASY-RES will develop basic business models based on detailed cost-functions per AS and on the estimation of the investments that are deferred when the new AS are offered by the DRES. Such investments are e.g. line reinforcements, protection system upgrades, use of capacitor or coil banks, reduction of conventional reserves, etc.

So far, suitable metrics have been developed for the proper measurement and quantification of the required AS. Moreover, expressions have been derived to evaluate the DRES converter losses due to reactive power exchange and a new voltage control scheme for MV networks has been proposed in recent scientific publications in the frame of EASY-RES.

Anticipated Impact

EASY-RES is expected to improve energy security by allowing very high RES penetration (up to 100%) in the grid with simultaneous decommission of large SG driven by fossil fuels. This is expected to have a great impact on the environment and will contribute to solving the global climate and energy challenges.

The high RES penetration will be demonstrated through the following technical goals:

• The average overall system inertia will be increased by at least 1.5% for every 10% increase in DRES penetration due to the DRES controllable inertia.
• For every 3 MW of DRES integrated into the system, more than 2.5 MW of conventional reserves will be decommitted.
• The extended DRES reactive power capability and the new controllable contribution to fault-clearing process are expected to make the ICA autonomous with respect to voltage regulation and protection coordination, while avoiding additional investments in grid reinforcement and new protection systems.

EASY-RES is expected to have economic impacts in the energy policy and markets, as well. The new metrics will enable the development of (i) viable business models between DSO/TSO; (ii) new AS markets within the ICAs in which DRES owners, including the small ones, can participate either alone or in aggregated communities.