Advances in Ancillary Services by DRES in Distribution Grids

Dear Colleagues,

In a future energy scenario with very high shares of renewables (up to 100%) in the electrical energy mix, system support functions that are provided today by synchronous generators (SG) will need to be provided by distributed renewable energy sources (DRES) or procured from third parties. DRES should significantly contribute to a more stable operation of the future energy system, allowing growing percentage of DRES to displace traditional SG-based dispatchable generation. The challenge is to define and develop system support functions or ancillary services (AS) for the contribution of DRES to stable and safe electrical energy system operations in the best technical and economic way.  It is reminded that the traditional large SGs exhibit inherently a number of significant properties that need to be provided in the future by the DRES individually or in aggregated form to keep the stable system operation at its current standards or improve it.

Such properties are:

• they exhibit inertia due to their rotational masses;
• they have grid forming capability allowing them to participate in black-starting processes;
• they can exchange controllable reactive power contributing, thus, to the voltage regulation;
• they produce symmetrical, fundamental-harmonic-only voltages maintaining thus the system power quality;
• they inject large currents, however, non-controllable, during faults enabling thus the fault-clearing with the current protection means.

In addition, these large units are generally dispatchable as they are associated with fossil-fuel, nuclear or hydro plants. For this reason, they are participating in the active power balancing process acting as Frequency Containment Reserves (FCR) and/or Frequency Restoration Reserves (FRR) and/or Replacement Reserves (RR). A number of the SG properties, such as the reactive power exchange, the black starting and the action as FCR/FRR/RR are traded as AS in the respective markets at transmission system level. Other properties, such as inertia, are considered as system support functions and are not remunerated.

DRES connected in the distribution system, particularly those that are converter-interfaced, are highly controllable, thereby they can provide a wide range of services to the distribution system and to the transmission system, provided these services are aggregated. However, in order for these services to be treated as AS and not only as system support functions, they have to be accurately measured and quantified in a unified way both at DRES level and in aggregated form at the Point of Interconnection (POI) of the distribution with the transmission system. An indicative but not exhaustive list of such services that can be offered by DRES is:

• provision of controllable and adjustable synthetic inertia;
• operation in frequency sensitive mode both upwards and downwards with controllable and adjustable droops and frequency deadbands;
• exchange of controllable reactive power in both over- and under-excited mode for voltage regulation purposes;
• control of the active power ramp rates both at DRES terminal and at the POI between the distribution and transmission grid;
• adjustable Fault-Ride-Through (FRT) capability and controllable injection of the currents during faults in the distribution system in order to preserve the selectivity of the currently used fault-protection systems;
• action as active harmonic filters in order to mitigate voltage harmonics within the distribution system.        

The aim of this Special Issue is to attract leading researchers in the area of possible AS that can be offered by DRES connected in the distribution system. Accepted contributions will include one or more of the following topics:

• The control of the power-electronic converters of DRES to offer AS similar to those of conventional SG. Of particular interest is the development of solutions for the measurement and quantification of these AS so that they can become tradable in AS markets established either at transmission or at distribution system level.
• Methods, to be used by future AS aggregators at distribution system level, so that the aggregated value of an AS at the POI of the distribution and transmission system, can be estimated based on the individual AS of each DRES considering also the distribution system topology and the type and location of the DRES within it.
• Methods, to be used by future AS aggregators, so that they can optimally allocated an AS agreed with the TSO to the individual DRES belonging in their portfolio considering also the distribution system topology and the type and location of the DRES within it.
• Methods for the quantification of the costs and benefits associated with an AS so that viable business models can be formulated around it.
• Design of the required ICT system at distribution grid level to control the DRES in order to offer AS in a coordinated way together with the measurement and secure accounting of the AS. Security issues associated with the ICT system are of large significance.
• Methods of controlling the DRES (both dispatchable and variable-output) within a distribution grid, so that the latter behaves like a VPP able to offer controllable AS to the transmission system. Of particular interest are AS in the transient or dynamic domain like, inertial response, fast frequency response, FCR/FRR, dynamic reactive power support.
• Methods of controlling the DRES (both dispatchable and variable-output) and dispatchable loads within a distribution grid, so that the latter exhibits black starting capability without the use of storage systems.

Prof. Dr. Charis S. Demoulias
Dr. Angelos K. Marnerides
Dr. Georgios C. Kryonidis
Dr. Kyriaki-Nefeli D. Malamaki
Guest Editors

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• Measurement and quantification of AS at distribution grid level.
• Operation of distribution grids as VPP
• Aggregation of AS
• Investment, operational and maintenance incremental cost of DRES when offering an AS.
• Distributed Renewable Energy Sources
• ICT systems in distribution grids
• Synthetic/virtual inertia
• Voltage regulation in distribution grids
• Power smoothing by DRES
• Active power ramp rate control by DRES
• Operation of variable-output DRES in frequency sensitive mode